src/gpu-compute/tlb_coalescer.cc - public/gem5 - Git at Google

 /*
  * Copyright (c) 2011-2015 Advanced Micro Devices, Inc.
  * All rights reserved.
  *
  * For use for simulation and test purposes only
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  * 1. Redistributions of source code must retain the above copyright notice,
  * this list of conditions and the following disclaimer.
  *
  * 2. Redistributions in binary form must reproduce the above copyright notice,
  * this list of conditions and the following disclaimer in the documentation
  * and/or other materials provided with the distribution.
  *
  * 3. Neither the name of the copyright holder nor the names of its
  * contributors may be used to endorse or promote products derived from this
  * software without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  * POSSIBILITY OF SUCH DAMAGE.
  */

 #include "gpu-compute/tlb_coalescer.hh"

 #include <cstring>

 #include "arch/x86/isa_traits.hh"
 #include "base/logging.hh"
 #include "debug/GPUTLB.hh"
 #include "sim/process.hh"

 TLBCoalescer::TLBCoalescer(const Params &p)
     : ClockedObject(p),
       TLBProbesPerCycle(p.probesPerCycle),
       coalescingWindow(p.coalescingWindow),
       disableCoalescing(p.disableCoalescing),
       probeTLBEvent([this]{ processProbeTLBEvent(); },
                     "Probe the TLB below",
                     false, Event::CPU_Tick_Pri),
       cleanupEvent([this]{ processCleanupEvent(); },
                    "Cleanup issuedTranslationsTable hashmap",
                    false, Event::Maximum_Pri),
       stats(this)
 {
     // create the response ports based on the number of connected ports
     for (size_t i = 0; i < p.port_cpu_side_ports_connection_count; ++i) {
         cpuSidePort.push_back(new CpuSidePort(csprintf("%s-port%d", name(), i),
                                               this, i));
     }

     // create the request ports based on the number of connected ports
     for (size_t i = 0; i < p.port_mem_side_ports_connection_count; ++i) {
         memSidePort.push_back(new MemSidePort(csprintf("%s-port%d", name(), i),
                                               this, i));
     }
 }

 Port &
 TLBCoalescer::getPort(const std::string &if_name, PortID idx)
 {
     if (if_name == "cpu_side_ports") {
         if (idx >= static_cast<PortID>(cpuSidePort.size())) {
             panic("TLBCoalescer::getPort: unknown index %d\n", idx);
         }

         return *cpuSidePort[idx];
     } else  if (if_name == "mem_side_ports") {
         if (idx >= static_cast<PortID>(memSidePort.size())) {
             panic("TLBCoalescer::getPort: unknown index %d\n", idx);
         }

         return *memSidePort[idx];
     } else {
         panic("TLBCoalescer::getPort: unknown port %s\n", if_name);
     }
 }

 /*
  * This method returns true if the <incoming_pkt>
  * can be coalesced with <coalesced_pkt> and false otherwise.
  * A given set of rules is checked.
  * The rules can potentially be modified based on the TLB level.
  */
 bool
 TLBCoalescer::canCoalesce(PacketPtr incoming_pkt, PacketPtr coalesced_pkt)
 {
     if (disableCoalescing)
         return false;

     TheISA::GpuTLB::TranslationState *incoming_state =
       safe_cast<TheISA::GpuTLB::TranslationState*>(incoming_pkt->senderState);

     TheISA::GpuTLB::TranslationState *coalesced_state =
      safe_cast<TheISA::GpuTLB::TranslationState*>(coalesced_pkt->senderState);

     // Rule 1: Coalesce requests only if they
     // fall within the same virtual page
     Addr incoming_virt_page_addr = roundDown(incoming_pkt->req->getVaddr(),
                                              X86ISA::PageBytes);

     Addr coalesced_virt_page_addr = roundDown(coalesced_pkt->req->getVaddr(),
                                               X86ISA::PageBytes);

     if (incoming_virt_page_addr != coalesced_virt_page_addr)
         return false;

     //* Rule 2: Coalesce requests only if they
     // share a TLB Mode, i.e. they are both read
     // or write requests.
     BaseTLB::Mode incoming_mode = incoming_state->tlbMode;
     BaseTLB::Mode coalesced_mode = coalesced_state->tlbMode;

     if (incoming_mode != coalesced_mode)
         return false;

     // when we can coalesce a packet update the reqCnt
     // that is the number of packets represented by
     // this coalesced packet
     if (!incoming_state->prefetch)
         coalesced_state->reqCnt.back() += incoming_state->reqCnt.back();

     return true;
 }

 /*
  * We need to update the physical addresses of all the translation requests
  * that were coalesced into the one that just returned.
  */
 void
 TLBCoalescer::updatePhysAddresses(PacketPtr pkt)
 {
     Addr virt_page_addr = roundDown(pkt->req->getVaddr(), X86ISA::PageBytes);

     DPRINTF(GPUTLB, "Update phys. addr. for %d coalesced reqs for page %#x\n",
             issuedTranslationsTable[virt_page_addr].size(), virt_page_addr);

     TheISA::GpuTLB::TranslationState *sender_state =
         safe_cast<TheISA::GpuTLB::TranslationState*>(pkt->senderState);

     TheISA::TlbEntry *tlb_entry = sender_state->tlbEntry;
     assert(tlb_entry);
     Addr first_entry_vaddr = tlb_entry->vaddr;
     Addr first_entry_paddr = tlb_entry->paddr;
     int page_size = tlb_entry->size();
     bool uncacheable = tlb_entry->uncacheable;
     int first_hit_level = sender_state->hitLevel;

     // Get the physical page address of the translated request
     // Using the page_size specified in the TLBEntry allows us
     // to support different page sizes.
     Addr phys_page_paddr = pkt->req->getPaddr();
     phys_page_paddr &= ~(page_size - 1);

     for (int i = 0; i < issuedTranslationsTable[virt_page_addr].size(); ++i) {
         PacketPtr local_pkt = issuedTranslationsTable[virt_page_addr][i];
         TheISA::GpuTLB::TranslationState *sender_state =
             safe_cast<TheISA::GpuTLB::TranslationState*>(
                     local_pkt->senderState);

         // we are sending the packet back, so pop the reqCnt associated
         // with this level in the TLB hiearchy
         if (!sender_state->prefetch)
             sender_state->reqCnt.pop_back();

         /*
          * Only the first packet from this coalesced request has been
          * translated. Grab the translated phys. page addr and update the
          * physical addresses of the remaining packets with the appropriate
          * page offsets.
          */
         if (i) {
             Addr paddr = phys_page_paddr;
             paddr |= (local_pkt->req->getVaddr() & (page_size - 1));
             local_pkt->req->setPaddr(paddr);

             if (uncacheable)
                 local_pkt->req->setFlags(Request::UNCACHEABLE);

             // update senderState->tlbEntry, so we can insert
             // the correct TLBEentry in the TLBs above.
             auto p = sender_state->tc->getProcessPtr();
             sender_state->tlbEntry =
                 new TheISA::TlbEntry(p->pid(), first_entry_vaddr,
                     first_entry_paddr, false, false);

             // update the hitLevel for all uncoalesced reqs
             // so that each packet knows where it hit
             // (used for statistics in the CUs)
             sender_state->hitLevel = first_hit_level;
         }

         ResponsePort *return_port = sender_state->ports.back();
         sender_state->ports.pop_back();

         // Translation is done - Convert to a response pkt if necessary and
         // send the translation back
         if (local_pkt->isRequest()) {
             local_pkt->makeTimingResponse();
         }

         return_port->sendTimingResp(local_pkt);
     }

     // schedule clean up for end of this cycle
     // This is a maximum priority event and must be on
     // the same cycle as GPUTLB cleanup event to prevent
     // race conditions with an IssueProbeEvent caused by
     // MemSidePort::recvReqRetry
     cleanupQueue.push(virt_page_addr);

     if (!cleanupEvent.scheduled())
         schedule(cleanupEvent, curTick());
 }

 // Receive translation requests, create a coalesced request,
 // and send them to the TLB (TLBProbesPerCycle)
 bool
 TLBCoalescer::CpuSidePort::recvTimingReq(PacketPtr pkt)
 {
     // first packet of a coalesced request
     PacketPtr first_packet = nullptr;
     // true if we are able to do coalescing
     bool didCoalesce = false;
     // number of coalesced reqs for a given window
     int coalescedReq_cnt = 0;

     TheISA::GpuTLB::TranslationState *sender_state =
         safe_cast<TheISA::GpuTLB::TranslationState*>(pkt->senderState);

     // push back the port to remember the path back
     sender_state->ports.push_back(this);

     bool update_stats = !sender_state->prefetch;

     if (update_stats) {
         // if reqCnt is empty then this packet does not represent
         // multiple uncoalesced reqs(pkts) but just a single pkt.
         // If it does though then the reqCnt for each level in the
         // hierarchy accumulates the total number of reqs this packet
         // represents
         int req_cnt = 1;

         if (!sender_state->reqCnt.empty())
             req_cnt = sender_state->reqCnt.back();

         sender_state->reqCnt.push_back(req_cnt);

         // update statistics
         coalescer->stats.uncoalescedAccesses++;
         req_cnt = sender_state->reqCnt.back();
         DPRINTF(GPUTLB, "receiving pkt w/ req_cnt %d\n", req_cnt);
         coalescer->stats.queuingCycles -= (curTick() * req_cnt);
         coalescer->stats.localqueuingCycles -= curTick();
     }

     // FIXME if you want to coalesce not based on the issueTime
     // of the packets (i.e., from the compute unit's perspective)
     // but based on when they reached this coalescer then
     // remove the following if statement and use curTick() or
     // coalescingWindow for the tick_index.
     if (!sender_state->issueTime)
        sender_state->issueTime = curTick();

     // The tick index is used as a key to the coalescerFIFO hashmap.
     // It is shared by all candidates that fall within the
     // given coalescingWindow.
     int64_t tick_index = sender_state->issueTime / coalescer->coalescingWindow;

     if (coalescer->coalescerFIFO.count(tick_index)) {
         coalescedReq_cnt = coalescer->coalescerFIFO[tick_index].size();
     }

     // see if we can coalesce the incoming pkt with another
     // coalesced request with the same tick_index
     for (int i = 0; i < coalescedReq_cnt; ++i) {
         first_packet = coalescer->coalescerFIFO[tick_index][i][0];

         if (coalescer->canCoalesce(pkt, first_packet)) {
             coalescer->coalescerFIFO[tick_index][i].push_back(pkt);

             DPRINTF(GPUTLB, "Coalesced req %i w/ tick_index %d has %d reqs\n",
                     i, tick_index,
                     coalescer->coalescerFIFO[tick_index][i].size());

             didCoalesce = true;
             break;
         }
     }

     // if this is the first request for this tick_index
     // or we did not manage to coalesce, update stats
     // and make necessary allocations.
     if (!coalescedReq_cnt || !didCoalesce) {
         if (update_stats)
             coalescer->stats.coalescedAccesses++;

         std::vector<PacketPtr> new_array;
         new_array.push_back(pkt);
         coalescer->coalescerFIFO[tick_index].push_back(new_array);

         DPRINTF(GPUTLB, "coalescerFIFO[%d] now has %d coalesced reqs after "
                 "push\n", tick_index,
                 coalescer->coalescerFIFO[tick_index].size());
     }

     //schedule probeTLBEvent next cycle to send the
     //coalesced requests to the TLB
     if (!coalescer->probeTLBEvent.scheduled()) {
         coalescer->schedule(coalescer->probeTLBEvent,
                 curTick() + coalescer->clockPeriod());
     }

     return true;
 }

 void
 TLBCoalescer::CpuSidePort::recvReqRetry()
 {
     panic("recvReqRetry called");
 }

 void
 TLBCoalescer::CpuSidePort::recvFunctional(PacketPtr pkt)
 {

     TheISA::GpuTLB::TranslationState *sender_state =
         safe_cast<TheISA::GpuTLB::TranslationState*>(pkt->senderState);

     bool update_stats = !sender_state->prefetch;

     if (update_stats)
         coalescer->stats.uncoalescedAccesses++;

     // If there is a pending timing request for this virtual address
     // print a warning message. This is a temporary caveat of
     // the current simulator where atomic and timing requests can
     // coexist. FIXME remove this check/warning in the future.
     Addr virt_page_addr = roundDown(pkt->req->getVaddr(), X86ISA::PageBytes);
     int map_count = coalescer->issuedTranslationsTable.count(virt_page_addr);

     if (map_count) {
         DPRINTF(GPUTLB, "Warning! Functional access to addr %#x sees timing "
                 "req. pending\n", virt_page_addr);
     }

     coalescer->memSidePort[0]->sendFunctional(pkt);
 }

 AddrRangeList
 TLBCoalescer::CpuSidePort::getAddrRanges() const
 {
     // currently not checked by the requestor
     AddrRangeList ranges;

     return ranges;
 }

 bool
 TLBCoalescer::MemSidePort::recvTimingResp(PacketPtr pkt)
 {
     // a translation completed and returned
     coalescer->updatePhysAddresses(pkt);

     return true;
 }

 void
 TLBCoalescer::MemSidePort::recvReqRetry()
 {
     //we've receeived a retry. Schedule a probeTLBEvent
     if (!coalescer->probeTLBEvent.scheduled())
         coalescer->schedule(coalescer->probeTLBEvent,
                 curTick() + coalescer->clockPeriod());
 }

 void
 TLBCoalescer::MemSidePort::recvFunctional(PacketPtr pkt)
 {
     fatal("Memory side recvFunctional() not implemented in TLB coalescer.\n");
 }

 /*
  * Here we scan the coalescer FIFO and issue the max
  * number of permitted probes to the TLB below. We
  * permit bypassing of coalesced requests for the same
  * tick_index.
  *
  * We do not access the next tick_index unless we've
  * drained the previous one. The coalesced requests
  * that are successfully sent are moved to the
  * issuedTranslationsTable table (the table which keeps
  * track of the outstanding reqs)
  */
 void
 TLBCoalescer::processProbeTLBEvent()
 {
     // number of TLB probes sent so far
     int sent_probes = 0;
     // rejected denotes a blocking event
     bool rejected = false;

     // It is set to true either when the recvTiming of the TLB below
     // returns false or when there is another outstanding request for the
     // same virt. page.

     DPRINTF(GPUTLB, "triggered TLBCoalescer %s\n", __func__);

     for (auto iter = coalescerFIFO.begin();
          iter != coalescerFIFO.end() && !rejected; ) {
         int coalescedReq_cnt = iter->second.size();
         int i = 0;
         int vector_index = 0;

         DPRINTF(GPUTLB, "coalescedReq_cnt is %d for tick_index %d\n",
                coalescedReq_cnt, iter->first);

         while (i < coalescedReq_cnt) {
             ++i;
             PacketPtr first_packet = iter->second[vector_index][0];

             // compute virtual page address for this request
             Addr virt_page_addr = roundDown(first_packet->req->getVaddr(),
                     X86ISA::PageBytes);

             // is there another outstanding request for the same page addr?
             int pending_reqs =
                 issuedTranslationsTable.count(virt_page_addr);

             if (pending_reqs) {
                 DPRINTF(GPUTLB, "Cannot issue - There are pending reqs for "
                         "page %#x\n", virt_page_addr);

                 ++vector_index;
                 rejected = true;

                 continue;
             }

             // send the coalesced request for virt_page_addr
             if (!memSidePort[0]->sendTimingReq(first_packet)) {
                 DPRINTF(GPUTLB, "Failed to send TLB request for page %#x\n",
                        virt_page_addr);

                 // No need for a retries queue since we are already buffering
                 // the coalesced request in coalescerFIFO.
                 rejected = true;
                 ++vector_index;
             } else {
                 TheISA::GpuTLB::TranslationState *tmp_sender_state =
                     safe_cast<TheISA::GpuTLB::TranslationState*>
                     (first_packet->senderState);

                 bool update_stats = !tmp_sender_state->prefetch;

                 if (update_stats) {
                     // req_cnt is total number of packets represented
                     // by the one we just sent counting all the way from
                     // the top of TLB hiearchy (i.e., from the CU)
                     int req_cnt = tmp_sender_state->reqCnt.back();
                     stats.queuingCycles += (curTick() * req_cnt);

                     DPRINTF(GPUTLB, "%s sending pkt w/ req_cnt %d\n",
                             name(), req_cnt);

                     // pkt_cnt is number of packets we coalesced into the one
                     // we just sent but only at this coalescer level
                     int pkt_cnt = iter->second[vector_index].size();
                     stats.localqueuingCycles += (curTick() * pkt_cnt);
                 }

                 DPRINTF(GPUTLB, "Successfully sent TLB request for page %#x",
                        virt_page_addr);

                 //copy coalescedReq to issuedTranslationsTable
                 issuedTranslationsTable[virt_page_addr]
                     = iter->second[vector_index];

                 //erase the entry of this coalesced req
                 iter->second.erase(iter->second.begin() + vector_index);

                 if (iter->second.empty())
                     assert(i == coalescedReq_cnt);

                 sent_probes++;
                 if (sent_probes == TLBProbesPerCycle)
                    return;
             }
         }

         //if there are no more coalesced reqs for this tick_index
         //erase the hash_map with the first iterator
         if (iter->second.empty()) {
             coalescerFIFO.erase(iter++);
         } else {
             ++iter;
         }
     }
 }

 void
 TLBCoalescer::processCleanupEvent()
 {
     while (!cleanupQueue.empty()) {
         Addr cleanup_addr = cleanupQueue.front();
         cleanupQueue.pop();
         issuedTranslationsTable.erase(cleanup_addr);

         DPRINTF(GPUTLB, "Cleanup - Delete coalescer entry with key %#x\n",
                 cleanup_addr);
     }
 }

 TLBCoalescer::TLBCoalescerStats::TLBCoalescerStats(Stats::Group *parent)
     : Stats::Group(parent),
       ADD_STAT(uncoalescedAccesses, "Number of uncoalesced TLB accesses"),
       ADD_STAT(coalescedAccesses, "Number of coalesced TLB accesses"),
       ADD_STAT(queuingCycles, "Number of cycles spent in queue"),
       ADD_STAT(localqueuingCycles,
                "Number of cycles spent in queue for all incoming reqs"),
       ADD_STAT(localLatency, "Avg. latency over all incoming pkts")
 {
     localLatency = localqueuingCycles / uncoalescedAccesses;
 }
	/*
	* Copyright (c) 2011-2015 Advanced Micro Devices, Inc.
	* All rights reserved.
	*
	* For use for simulation and test purposes only
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions are met:
	*
	* 1. Redistributions of source code must retain the above copyright notice,
	* this list of conditions and the following disclaimer.
	*
	* 2. Redistributions in binary form must reproduce the above copyright notice,
	* this list of conditions and the following disclaimer in the documentation
	* and/or other materials provided with the distribution.
	*
	* 3. Neither the name of the copyright holder nor the names of its
	* contributors may be used to endorse or promote products derived from this
	* software without specific prior written permission.
	*
	* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
	* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
	* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
	* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
	* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
	* POSSIBILITY OF SUCH DAMAGE.
	*/

	#include "gpu-compute/tlb_coalescer.hh"

	#include <cstring>

	#include "arch/x86/isa_traits.hh"
	#include "base/logging.hh"
	#include "debug/GPUTLB.hh"
	#include "sim/process.hh"

	TLBCoalescer::TLBCoalescer(const Params &p)
	: ClockedObject(p),
	TLBProbesPerCycle(p.probesPerCycle),
	coalescingWindow(p.coalescingWindow),
	disableCoalescing(p.disableCoalescing),
	probeTLBEvent([this]{ processProbeTLBEvent(); },
	"Probe the TLB below",
	false, Event::CPU_Tick_Pri),
	cleanupEvent([this]{ processCleanupEvent(); },
	"Cleanup issuedTranslationsTable hashmap",
	false, Event::Maximum_Pri),
	stats(this)
	{
	// create the response ports based on the number of connected ports
	for (size_t i = 0; i < p.port_cpu_side_ports_connection_count; ++i) {
	cpuSidePort.push_back(new CpuSidePort(csprintf("%s-port%d", name(), i),
	this, i));
	}

	// create the request ports based on the number of connected ports
	for (size_t i = 0; i < p.port_mem_side_ports_connection_count; ++i) {
	memSidePort.push_back(new MemSidePort(csprintf("%s-port%d", name(), i),
	this, i));
	}
	}

	Port &
	TLBCoalescer::getPort(const std::string &if_name, PortID idx)
	{
	if (if_name == "cpu_side_ports") {
	if (idx >= static_cast<PortID>(cpuSidePort.size())) {
	panic("TLBCoalescer::getPort: unknown index %d\n", idx);
	}

	return *cpuSidePort[idx];
	} else if (if_name == "mem_side_ports") {
	if (idx >= static_cast<PortID>(memSidePort.size())) {
	panic("TLBCoalescer::getPort: unknown index %d\n", idx);
	}

	return *memSidePort[idx];
	} else {
	panic("TLBCoalescer::getPort: unknown port %s\n", if_name);
	}
	}

	/*
	* This method returns true if the <incoming_pkt>
	* can be coalesced with <coalesced_pkt> and false otherwise.
	* A given set of rules is checked.
	* The rules can potentially be modified based on the TLB level.
	*/
	bool
	TLBCoalescer::canCoalesce(PacketPtr incoming_pkt, PacketPtr coalesced_pkt)
	{
	if (disableCoalescing)
	return false;

	TheISA::GpuTLB::TranslationState *incoming_state =
	safe_cast<TheISA::GpuTLB::TranslationState*>(incoming_pkt->senderState);

	TheISA::GpuTLB::TranslationState *coalesced_state =
	safe_cast<TheISA::GpuTLB::TranslationState*>(coalesced_pkt->senderState);

	// Rule 1: Coalesce requests only if they
	// fall within the same virtual page
	Addr incoming_virt_page_addr = roundDown(incoming_pkt->req->getVaddr(),
	X86ISA::PageBytes);

	Addr coalesced_virt_page_addr = roundDown(coalesced_pkt->req->getVaddr(),
	X86ISA::PageBytes);

	if (incoming_virt_page_addr != coalesced_virt_page_addr)
	return false;

	//* Rule 2: Coalesce requests only if they
	// share a TLB Mode, i.e. they are both read
	// or write requests.
	BaseTLB::Mode incoming_mode = incoming_state->tlbMode;
	BaseTLB::Mode coalesced_mode = coalesced_state->tlbMode;

	if (incoming_mode != coalesced_mode)
	return false;

	// when we can coalesce a packet update the reqCnt
	// that is the number of packets represented by
	// this coalesced packet
	if (!incoming_state->prefetch)
	coalesced_state->reqCnt.back() += incoming_state->reqCnt.back();

	return true;
	}

	/*
	* We need to update the physical addresses of all the translation requests
	* that were coalesced into the one that just returned.
	*/
	void
	TLBCoalescer::updatePhysAddresses(PacketPtr pkt)
	{
	Addr virt_page_addr = roundDown(pkt->req->getVaddr(), X86ISA::PageBytes);

	DPRINTF(GPUTLB, "Update phys. addr. for %d coalesced reqs for page %#x\n",
	issuedTranslationsTable[virt_page_addr].size(), virt_page_addr);

	TheISA::GpuTLB::TranslationState *sender_state =
	safe_cast<TheISA::GpuTLB::TranslationState*>(pkt->senderState);

	TheISA::TlbEntry *tlb_entry = sender_state->tlbEntry;
	assert(tlb_entry);
	Addr first_entry_vaddr = tlb_entry->vaddr;
	Addr first_entry_paddr = tlb_entry->paddr;
	int page_size = tlb_entry->size();
	bool uncacheable = tlb_entry->uncacheable;
	int first_hit_level = sender_state->hitLevel;

	// Get the physical page address of the translated request
	// Using the page_size specified in the TLBEntry allows us
	// to support different page sizes.
	Addr phys_page_paddr = pkt->req->getPaddr();
	phys_page_paddr &= ~(page_size - 1);

	for (int i = 0; i < issuedTranslationsTable[virt_page_addr].size(); ++i) {
	PacketPtr local_pkt = issuedTranslationsTable[virt_page_addr][i];
	TheISA::GpuTLB::TranslationState *sender_state =
	safe_cast<TheISA::GpuTLB::TranslationState*>(
	local_pkt->senderState);

	// we are sending the packet back, so pop the reqCnt associated
	// with this level in the TLB hiearchy
	if (!sender_state->prefetch)
	sender_state->reqCnt.pop_back();

	/*
	* Only the first packet from this coalesced request has been
	* translated. Grab the translated phys. page addr and update the
	* physical addresses of the remaining packets with the appropriate
	* page offsets.
	*/
	if (i) {
	Addr paddr = phys_page_paddr;
	paddr \|= (local_pkt->req->getVaddr() & (page_size - 1));
	local_pkt->req->setPaddr(paddr);

	if (uncacheable)
	local_pkt->req->setFlags(Request::UNCACHEABLE);

	// update senderState->tlbEntry, so we can insert
	// the correct TLBEentry in the TLBs above.
	auto p = sender_state->tc->getProcessPtr();
	sender_state->tlbEntry =
	new TheISA::TlbEntry(p->pid(), first_entry_vaddr,
	first_entry_paddr, false, false);

	// update the hitLevel for all uncoalesced reqs
	// so that each packet knows where it hit
	// (used for statistics in the CUs)
	sender_state->hitLevel = first_hit_level;
	}

	ResponsePort *return_port = sender_state->ports.back();
	sender_state->ports.pop_back();

	// Translation is done - Convert to a response pkt if necessary and
	// send the translation back
	if (local_pkt->isRequest()) {
	local_pkt->makeTimingResponse();
	}

	return_port->sendTimingResp(local_pkt);
	}

	// schedule clean up for end of this cycle
	// This is a maximum priority event and must be on
	// the same cycle as GPUTLB cleanup event to prevent
	// race conditions with an IssueProbeEvent caused by
	// MemSidePort::recvReqRetry
	cleanupQueue.push(virt_page_addr);

	if (!cleanupEvent.scheduled())
	schedule(cleanupEvent, curTick());
	}

	// Receive translation requests, create a coalesced request,
	// and send them to the TLB (TLBProbesPerCycle)
	bool
	TLBCoalescer::CpuSidePort::recvTimingReq(PacketPtr pkt)
	{
	// first packet of a coalesced request
	PacketPtr first_packet = nullptr;
	// true if we are able to do coalescing
	bool didCoalesce = false;
	// number of coalesced reqs for a given window
	int coalescedReq_cnt = 0;

	TheISA::GpuTLB::TranslationState *sender_state =
	safe_cast<TheISA::GpuTLB::TranslationState*>(pkt->senderState);

	// push back the port to remember the path back
	sender_state->ports.push_back(this);

	bool update_stats = !sender_state->prefetch;

	if (update_stats) {
	// if reqCnt is empty then this packet does not represent
	// multiple uncoalesced reqs(pkts) but just a single pkt.
	// If it does though then the reqCnt for each level in the
	// hierarchy accumulates the total number of reqs this packet
	// represents
	int req_cnt = 1;

	if (!sender_state->reqCnt.empty())
	req_cnt = sender_state->reqCnt.back();

	sender_state->reqCnt.push_back(req_cnt);

	// update statistics
	coalescer->stats.uncoalescedAccesses++;
	req_cnt = sender_state->reqCnt.back();
	DPRINTF(GPUTLB, "receiving pkt w/ req_cnt %d\n", req_cnt);
	coalescer->stats.queuingCycles -= (curTick() * req_cnt);
	coalescer->stats.localqueuingCycles -= curTick();
	}

	// FIXME if you want to coalesce not based on the issueTime
	// of the packets (i.e., from the compute unit's perspective)
	// but based on when they reached this coalescer then
	// remove the following if statement and use curTick() or
	// coalescingWindow for the tick_index.
	if (!sender_state->issueTime)
	sender_state->issueTime = curTick();

	// The tick index is used as a key to the coalescerFIFO hashmap.
	// It is shared by all candidates that fall within the
	// given coalescingWindow.
	int64_t tick_index = sender_state->issueTime / coalescer->coalescingWindow;

	if (coalescer->coalescerFIFO.count(tick_index)) {
	coalescedReq_cnt = coalescer->coalescerFIFO[tick_index].size();
	}

	// see if we can coalesce the incoming pkt with another
	// coalesced request with the same tick_index
	for (int i = 0; i < coalescedReq_cnt; ++i) {
	first_packet = coalescer->coalescerFIFO[tick_index][i][0];

	if (coalescer->canCoalesce(pkt, first_packet)) {
	coalescer->coalescerFIFO[tick_index][i].push_back(pkt);

	DPRINTF(GPUTLB, "Coalesced req %i w/ tick_index %d has %d reqs\n",
	i, tick_index,
	coalescer->coalescerFIFO[tick_index][i].size());

	didCoalesce = true;
	break;
	}
	}

	// if this is the first request for this tick_index
	// or we did not manage to coalesce, update stats
	// and make necessary allocations.
	if (!coalescedReq_cnt \|\| !didCoalesce) {
	if (update_stats)
	coalescer->stats.coalescedAccesses++;

	std::vector<PacketPtr> new_array;
	new_array.push_back(pkt);
	coalescer->coalescerFIFO[tick_index].push_back(new_array);

	DPRINTF(GPUTLB, "coalescerFIFO[%d] now has %d coalesced reqs after "
	"push\n", tick_index,
	coalescer->coalescerFIFO[tick_index].size());
	}

	//schedule probeTLBEvent next cycle to send the
	//coalesced requests to the TLB
	if (!coalescer->probeTLBEvent.scheduled()) {
	coalescer->schedule(coalescer->probeTLBEvent,
	curTick() + coalescer->clockPeriod());
	}

	return true;
	}

	void
	TLBCoalescer::CpuSidePort::recvReqRetry()
	{
	panic("recvReqRetry called");
	}

	void
	TLBCoalescer::CpuSidePort::recvFunctional(PacketPtr pkt)
	{

	TheISA::GpuTLB::TranslationState *sender_state =
	safe_cast<TheISA::GpuTLB::TranslationState*>(pkt->senderState);

	bool update_stats = !sender_state->prefetch;

	if (update_stats)
	coalescer->stats.uncoalescedAccesses++;

	// If there is a pending timing request for this virtual address
	// print a warning message. This is a temporary caveat of
	// the current simulator where atomic and timing requests can
	// coexist. FIXME remove this check/warning in the future.
	Addr virt_page_addr = roundDown(pkt->req->getVaddr(), X86ISA::PageBytes);
	int map_count = coalescer->issuedTranslationsTable.count(virt_page_addr);

	if (map_count) {
	DPRINTF(GPUTLB, "Warning! Functional access to addr %#x sees timing "
	"req. pending\n", virt_page_addr);
	}

	coalescer->memSidePort[0]->sendFunctional(pkt);
	}

	AddrRangeList
	TLBCoalescer::CpuSidePort::getAddrRanges() const
	{
	// currently not checked by the requestor
	AddrRangeList ranges;

	return ranges;
	}

	bool
	TLBCoalescer::MemSidePort::recvTimingResp(PacketPtr pkt)
	{
	// a translation completed and returned
	coalescer->updatePhysAddresses(pkt);

	return true;
	}

	void
	TLBCoalescer::MemSidePort::recvReqRetry()
	{
	//we've receeived a retry. Schedule a probeTLBEvent
	if (!coalescer->probeTLBEvent.scheduled())
	coalescer->schedule(coalescer->probeTLBEvent,
	curTick() + coalescer->clockPeriod());
	}

	void
	TLBCoalescer::MemSidePort::recvFunctional(PacketPtr pkt)
	{
	fatal("Memory side recvFunctional() not implemented in TLB coalescer.\n");
	}

	/*
	* Here we scan the coalescer FIFO and issue the max
	* number of permitted probes to the TLB below. We
	* permit bypassing of coalesced requests for the same
	* tick_index.
	*
	* We do not access the next tick_index unless we've
	* drained the previous one. The coalesced requests
	* that are successfully sent are moved to the
	* issuedTranslationsTable table (the table which keeps
	* track of the outstanding reqs)
	*/
	void
	TLBCoalescer::processProbeTLBEvent()
	{
	// number of TLB probes sent so far
	int sent_probes = 0;
	// rejected denotes a blocking event
	bool rejected = false;

	// It is set to true either when the recvTiming of the TLB below
	// returns false or when there is another outstanding request for the
	// same virt. page.

	DPRINTF(GPUTLB, "triggered TLBCoalescer %s\n", __func__);

	for (auto iter = coalescerFIFO.begin();
	iter != coalescerFIFO.end() && !rejected; ) {
	int coalescedReq_cnt = iter->second.size();
	int i = 0;
	int vector_index = 0;

	DPRINTF(GPUTLB, "coalescedReq_cnt is %d for tick_index %d\n",
	coalescedReq_cnt, iter->first);

	while (i < coalescedReq_cnt) {
	++i;
	PacketPtr first_packet = iter->second[vector_index][0];

	// compute virtual page address for this request
	Addr virt_page_addr = roundDown(first_packet->req->getVaddr(),
	X86ISA::PageBytes);

	// is there another outstanding request for the same page addr?
	int pending_reqs =
	issuedTranslationsTable.count(virt_page_addr);

	if (pending_reqs) {
	DPRINTF(GPUTLB, "Cannot issue - There are pending reqs for "
	"page %#x\n", virt_page_addr);

	++vector_index;
	rejected = true;

	continue;
	}

	// send the coalesced request for virt_page_addr
	if (!memSidePort[0]->sendTimingReq(first_packet)) {
	DPRINTF(GPUTLB, "Failed to send TLB request for page %#x\n",
	virt_page_addr);

	// No need for a retries queue since we are already buffering
	// the coalesced request in coalescerFIFO.
	rejected = true;
	++vector_index;
	} else {
	TheISA::GpuTLB::TranslationState *tmp_sender_state =
	safe_cast<TheISA::GpuTLB::TranslationState*>
	(first_packet->senderState);

	bool update_stats = !tmp_sender_state->prefetch;

	if (update_stats) {
	// req_cnt is total number of packets represented
	// by the one we just sent counting all the way from
	// the top of TLB hiearchy (i.e., from the CU)
	int req_cnt = tmp_sender_state->reqCnt.back();
	stats.queuingCycles += (curTick() * req_cnt);

	DPRINTF(GPUTLB, "%s sending pkt w/ req_cnt %d\n",
	name(), req_cnt);

	// pkt_cnt is number of packets we coalesced into the one
	// we just sent but only at this coalescer level
	int pkt_cnt = iter->second[vector_index].size();
	stats.localqueuingCycles += (curTick() * pkt_cnt);
	}

	DPRINTF(GPUTLB, "Successfully sent TLB request for page %#x",
	virt_page_addr);

	//copy coalescedReq to issuedTranslationsTable
	issuedTranslationsTable[virt_page_addr]
	= iter->second[vector_index];

	//erase the entry of this coalesced req
	iter->second.erase(iter->second.begin() + vector_index);

	if (iter->second.empty())
	assert(i == coalescedReq_cnt);

	sent_probes++;
	if (sent_probes == TLBProbesPerCycle)
	return;
	}
	}

	//if there are no more coalesced reqs for this tick_index
	//erase the hash_map with the first iterator
	if (iter->second.empty()) {
	coalescerFIFO.erase(iter++);
	} else {
	++iter;
	}
	}
	}

	void
	TLBCoalescer::processCleanupEvent()
	{
	while (!cleanupQueue.empty()) {
	Addr cleanup_addr = cleanupQueue.front();
	cleanupQueue.pop();
	issuedTranslationsTable.erase(cleanup_addr);

	DPRINTF(GPUTLB, "Cleanup - Delete coalescer entry with key %#x\n",
	cleanup_addr);
	}
	}

	TLBCoalescer::TLBCoalescerStats::TLBCoalescerStats(Stats::Group *parent)
	: Stats::Group(parent),
	ADD_STAT(uncoalescedAccesses, "Number of uncoalesced TLB accesses"),
	ADD_STAT(coalescedAccesses, "Number of coalesced TLB accesses"),
	ADD_STAT(queuingCycles, "Number of cycles spent in queue"),
	ADD_STAT(localqueuingCycles,
	"Number of cycles spent in queue for all incoming reqs"),
	ADD_STAT(localLatency, "Avg. latency over all incoming pkts")
	{
	localLatency = localqueuingCycles / uncoalescedAccesses;
	}