RubyPrefetcher.cc

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/*
 * Copyright (c) 2020 Inria
 * Copyright (c) 2020 ARM Limited
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 * Copyright (c) 1999-2012 Mark D. Hill and David A. Wood
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#include "mem/ruby/structures/RubyPrefetcher.hh"
 
#include <cassert>
 
#include "base/bitfield.hh"
#include "debug/RubyPrefetcher.hh"
#include "mem/ruby/slicc_interface/RubySlicc_ComponentMapping.hh"
#include "mem/ruby/system/RubySystem.hh"
 
RubyPrefetcher::RubyPrefetcher(const Params &p)
    : SimObject(p), m_num_streams(p.num_streams),
    m_array(p.num_streams), m_train_misses(p.train_misses),
    m_num_startup_pfs(p.num_startup_pfs),
    unitFilter(p.unit_filter),
    negativeFilter(p.unit_filter),
    nonUnitFilter(p.nonunit_filter),
    m_prefetch_cross_pages(p.cross_page),
    m_page_shift(p.sys->getPageShift()),
    rubyPrefetcherStats(this)
{
    assert(m_num_streams > 0);
    assert(m_num_startup_pfs <= MAX_PF_INFLIGHT);
}
 
RubyPrefetcher::
RubyPrefetcherStats::RubyPrefetcherStats(Stats::Group *parent)
    : Stats::Group(parent, "RubyPrefetcher"),
      ADD_STAT(numMissObserved, "Number of misses observed"),
      ADD_STAT(numAllocatedStreams, "Number of streams allocated for "
                                    "prefetching"),
      ADD_STAT(numPrefetchRequested, "Number of prefetch requests made"),
      ADD_STAT(numHits, "Number of prefetched blocks accessed "
                        "(for the first time)"),
      ADD_STAT(numPartialHits, "Number of misses observed for a block being "
                               "prefetched"),
      ADD_STAT(numPagesCrossed, "Number of prefetches across pages"),
      ADD_STAT(numMissedPrefetchedBlocks, "Number of misses for blocks that "
                                          "were prefetched, yet missed")
{
}
 
void
RubyPrefetcher::observeMiss(Addr address, const RubyRequestType& type)
{
    DPRINTF(RubyPrefetcher, "Observed miss for %#x\n", address);
    Addr line_addr = makeLineAddress(address);
    rubyPrefetcherStats.numMissObserved++;
 
    // check to see if we have already issued a prefetch for this block
    uint32_t index = 0;
    PrefetchEntry *pfEntry = getPrefetchEntry(line_addr, index);
    if (pfEntry != NULL) {
        if (pfEntry->requestIssued[index]) {
            if (pfEntry->requestCompleted[index]) {
                // We prefetched too early and now the prefetch block no
                // longer exists in the cache
                rubyPrefetcherStats.numMissedPrefetchedBlocks++;
                return;
            } else {
                // The controller has issued the prefetch request,
                // but the request for the block arrived earlier.
                rubyPrefetcherStats.numPartialHits++;
                observePfMiss(line_addr);
                return;
            }
        } else {
            // The request is still in the prefetch queue of the controller.
            // Or was evicted because of other requests.
            return;
        }
    }
 
    // Check if address is in any of the stride filters
    if (accessUnitFilter(&unitFilter, line_addr, 1, type)) {
        DPRINTF(RubyPrefetcher, "  *** hit in unit stride buffer\n");
        return;
    }
    if (accessUnitFilter(&negativeFilter, line_addr, -1, type)) {
        DPRINTF(RubyPrefetcher, "  *** hit in unit negative unit buffer\n");
        return;
    }
    if (accessNonunitFilter(line_addr, type)) {
        DPRINTF(RubyPrefetcher, "  *** hit in non-unit stride buffer\n");
        return;
    }
}
 
void
RubyPrefetcher::observePfMiss(Addr address)
{
    rubyPrefetcherStats.numPartialHits++;
    DPRINTF(RubyPrefetcher, "Observed partial hit for %#x\n", address);
    issueNextPrefetch(address, NULL);
}
 
void
RubyPrefetcher::observePfHit(Addr address)
{
    rubyPrefetcherStats.numHits++;
    DPRINTF(RubyPrefetcher, "Observed hit for %#x\n", address);
    issueNextPrefetch(address, NULL);
}
 
void
RubyPrefetcher::issueNextPrefetch(Addr address, PrefetchEntry *stream)
{
    // get our corresponding stream fetcher
    if (stream == NULL) {
        uint32_t index = 0;
        stream = getPrefetchEntry(address, index);
    }
 
    // if (for some reason), this stream is unallocated, return.
    if (stream == NULL) {
        DPRINTF(RubyPrefetcher, "Unallocated stream, returning\n");
        return;
    }
 
    // extend this prefetching stream by 1 (or more)
    Addr page_addr = pageAddress(stream->m_address);
    Addr line_addr = makeNextStrideAddress(stream->m_address,
                                         stream->m_stride);
 
    // possibly stop prefetching at page boundaries
    if (page_addr != pageAddress(line_addr)) {
        if (!m_prefetch_cross_pages) {
            // Deallocate the stream since we are not prefetching
            // across page boundries
            stream->m_is_valid = false;
            return;
        }
        rubyPrefetcherStats.numPagesCrossed++;
    }
 
    // launch next prefetch
    rubyPrefetcherStats.numPrefetchRequested++;
    stream->m_address = line_addr;
    stream->m_use_time = m_controller->curCycle();
    DPRINTF(RubyPrefetcher, "Requesting prefetch for %#x\n", line_addr);
    m_controller->enqueuePrefetch(line_addr, stream->m_type);
}
 
uint32_t
RubyPrefetcher::getLRUindex(void)
{
    uint32_t lru_index = 0;
    Cycles lru_access = m_array[lru_index].m_use_time;
 
    for (uint32_t i = 0; i < m_num_streams; i++) {
        if (!m_array[i].m_is_valid) {
            return i;
        }
        if (m_array[i].m_use_time < lru_access) {
            lru_access = m_array[i].m_use_time;
            lru_index = i;
        }
    }
 
    return lru_index;
}
 
void
RubyPrefetcher::initializeStream(Addr address, int stride,
     uint32_t index, const RubyRequestType& type)
{
    rubyPrefetcherStats.numAllocatedStreams++;
 
    // initialize the stream prefetcher
    PrefetchEntry *mystream = &(m_array[index]);
    mystream->m_address = makeLineAddress(address);
    mystream->m_stride = stride;
    mystream->m_use_time = m_controller->curCycle();
    mystream->m_is_valid = true;
    mystream->m_type = type;
 
    // create a number of initial prefetches for this stream
    Addr page_addr = pageAddress(mystream->m_address);
    Addr line_addr = makeLineAddress(mystream->m_address);
 
    // insert a number of prefetches into the prefetch table
    for (int k = 0; k < m_num_startup_pfs; k++) {
        line_addr = makeNextStrideAddress(line_addr, stride);
        // possibly stop prefetching at page boundaries
        if (page_addr != pageAddress(line_addr)) {
            if (!m_prefetch_cross_pages) {
                // deallocate this stream prefetcher
                mystream->m_is_valid = false;
                return;
            }
            rubyPrefetcherStats.numPagesCrossed++;
        }
 
        // launch prefetch
        rubyPrefetcherStats.numPrefetchRequested++;
        DPRINTF(RubyPrefetcher, "Requesting prefetch for %#x\n", line_addr);
        m_controller->enqueuePrefetch(line_addr, m_array[index].m_type);
    }
 
    // update the address to be the last address prefetched
    mystream->m_address = line_addr;
}
 
PrefetchEntry *
RubyPrefetcher::getPrefetchEntry(Addr address, uint32_t &index)
{
    // search all streams for a match
    for (int i = 0; i < m_num_streams; i++) {
        // search all the outstanding prefetches for this stream
        if (m_array[i].m_is_valid) {
            for (int j = 0; j < m_num_startup_pfs; j++) {
                if (makeNextStrideAddress(m_array[i].m_address,
                    -(m_array[i].m_stride*j)) == address) {
                    return &(m_array[i]);
                }
            }
        }
    }
    return NULL;
}
 
bool
RubyPrefetcher::accessUnitFilter(CircularQueue<UnitFilterEntry>* const filter,
    Addr line_addr, int stride, const RubyRequestType& type)
{
    for (auto& entry : *filter) {
        if (entry.addr == line_addr) {
            entry.addr = makeNextStrideAddress(entry.addr, stride);
            entry.hits++;
            if (entry.hits >= m_train_misses) {
                // Allocate a new prefetch stream
                initializeStream(line_addr, stride, getLRUindex(), type);
            }
            return true;
        }
    }
 
    // Enter this address in the filter
    filter->push_back(UnitFilterEntry(
        makeNextStrideAddress(line_addr, stride)));
 
    return false;
}
 
bool
RubyPrefetcher::accessNonunitFilter(Addr line_addr,
    const RubyRequestType& type)
{
    Addr page_addr = pageAddress(line_addr);
 
    for (auto& entry : nonUnitFilter) {
        if (pageAddress(entry.addr) == page_addr) {
            // hit in the non-unit filter
            // compute the actual stride (for this reference)
            int delta = line_addr - entry.addr;
 
            if (delta != 0) {
                // no zero stride prefetches
                // check that the stride matches (for the last N times)
                if (delta == entry.stride) {
                    // -> stride hit
                    // increment count (if > 2) allocate stream
                    entry.hits++;
                    if (entry.hits > m_train_misses) {
                        // This stride HAS to be the multiplicative constant of
                        // dataBlockBytes (bc makeNextStrideAddress is
                        // calculated based on this multiplicative constant!)
                        const int stride = entry.stride /
                            RubySystem::getBlockSizeBytes();
 
                        // clear this filter entry
                        entry.clear();
 
                        initializeStream(line_addr, stride, getLRUindex(),
                            type);
                    }
                } else {
                    // If delta didn't match reset entry's hit count
                    entry.hits = 0;
                }
 
                // update the last address seen & the stride
                entry.addr = line_addr;
                entry.stride = delta;
                return true;
            } else {
                return false;
            }
        }
    }
 
    // not found: enter this address in the table
    nonUnitFilter.push_back(NonUnitFilterEntry(line_addr));
 
    return false;
}
 
void
RubyPrefetcher::print(std::ostream& out) const
{
    out << name() << " Prefetcher State\n";
    // print out unit filter
    out << "unit table:\n";
    for (const auto& entry : unitFilter) {
        out << entry.addr << std::endl;
    }
 
    out << "negative table:\n";
    for (const auto& entry : negativeFilter) {
        out << entry.addr << std::endl;
    }
 
    // print out non-unit stride filter
    out << "non-unit table:\n";
    for (const auto& entry : nonUnitFilter) {
        out << entry.addr << " "
            << entry.stride << " "
            << entry.hits << std::endl;
    }
 
    // print out allocated stream buffers
    out << "streams:\n";
    for (int i = 0; i < m_num_streams; i++) {
        out << m_array[i].m_address << " "
            << m_array[i].m_stride << " "
            << m_array[i].m_is_valid << " "
            << m_array[i].m_use_time << std::endl;
    }
}
 
Addr
RubyPrefetcher::pageAddress(Addr addr) const
{
    return mbits<Addr>(addr, 63, m_page_shift);
}