[compiler-rt] [scudo] Add partial chunk heuristic to retrieval algorithm. (PR #105009)
Joshua Baehring via llvm-commits
llvm-commits at lists.llvm.org
Tue Aug 20 08:24:33 PDT 2024
https://github.com/JoshuaMBa created https://github.com/llvm/llvm-project/pull/105009
Previously the secondary cache retrieval algorithm would not allow retrievals of memory chunks where the number of unused bytes would be greater than than `MaxUnusedCachePages * PageSize` bytes. This meant that even if a memory chunk satisfied the requirements of the optimal fit algorithm, it may not be returned. This remains true if memory tagging is enabled. However, if memory tagging is disabled, a new heuristic has been put in place. Specifically, If a memory chunk is a non-optimal fit, the cache retrieval algorithm will attempt to release the excess memory to force a cache hit while keeping RSS down.
In the event that a memory chunk is a non-optimal fit, the retrieval algorithm will release excess memory as long as the amount of memory to be released is less than or equal to 4 Pages. If the amount of memory to be released exceeds 4 Pages, the retrieval algorithm will not consider that cached memory chunk valid for retrieval.
>From 665d3a0163d619d6f4662ddb5cf2bd458d773a54 Mon Sep 17 00:00:00 2001
From: Joshua Baehring <jmbaehring at google.com>
Date: Tue, 20 Aug 2024 06:18:14 +0000
Subject: [PATCH] [scudo] Add partial chunk heuristic to retrieval algorithm.
Previously the secondary cache retrieval algorithm would not allow
retrievals of memory chunks where the number of unused bytes would
be greater than than `MaxUnusedCachePages * PageSize` bytes. This
meant that even if a memory chunk satisfied the requirements of the
optimal fit algorithm, it may not be returned. This remains true if
memory tagging is enabled. However, if memory tagging is disabled,
a new heuristic has been put in place. Specifically, If a memory
chunk is a non-optimal fit, the cache retrieval algorithm will
attempt to release the excess memory to force a cache hit while
keeping RSS down.
In the event that a memory chunk is a non-optimal fit, the retrieval
algorithm will release excess memory as long as the amount of memory
to be released is less than or equal to 4 Pages. If the amount of memory
to be released exceeds 4 Pages, the retrieval algorithm will not
consider that cached memory chunk valid for retrieval.
---
compiler-rt/lib/scudo/standalone/secondary.h | 125 +++++++++++++-----
.../scudo/standalone/tests/secondary_test.cpp | 38 +++++-
2 files changed, 129 insertions(+), 34 deletions(-)
diff --git a/compiler-rt/lib/scudo/standalone/secondary.h b/compiler-rt/lib/scudo/standalone/secondary.h
index 27f8697db7838f..2adea34c13b426 100644
--- a/compiler-rt/lib/scudo/standalone/secondary.h
+++ b/compiler-rt/lib/scudo/standalone/secondary.h
@@ -72,6 +72,14 @@ namespace {
struct CachedBlock {
static constexpr u16 CacheIndexMax = UINT16_MAX;
static constexpr u16 InvalidEntry = CacheIndexMax;
+ // * MaxCachePagesToRelease default is currently 4
+ // - We arrived at this value after noticing that mapping
+ // in larger memory regions performs better than releasing
+ // memory and forcing a cache hit. According to the data,
+ // it suggests that beyond 16KB, the release execution time is
+ // longer than the map execution time. In this way, the default
+ // is dependent on the platform.
+ static constexpr uptr MaxCachePagesToRelease = 4U;
uptr CommitBase = 0;
uptr CommitSize = 0;
@@ -90,8 +98,9 @@ struct CachedBlock {
template <typename Config> class MapAllocatorNoCache {
public:
void init(UNUSED s32 ReleaseToOsInterval) {}
- CachedBlock retrieve(UNUSED uptr Size, UNUSED uptr Alignment,
- UNUSED uptr HeadersSize, UNUSED uptr &EntryHeaderPos) {
+ CachedBlock retrieve(UNUSED uptr MaxAllowedFragmentedBytes, UNUSED uptr Size,
+ UNUSED uptr Alignment, UNUSED uptr HeadersSize,
+ UNUSED uptr &EntryHeaderPos) {
return {};
}
void store(UNUSED Options Options, UNUSED uptr CommitBase,
@@ -334,13 +343,14 @@ class MapAllocatorCache {
}
}
- CachedBlock retrieve(uptr Size, uptr Alignment, uptr HeadersSize,
- uptr &EntryHeaderPos) EXCLUDES(Mutex) {
+ CachedBlock retrieve(uptr MaxAllowedFragmentedBytes, uptr Size,
+ uptr Alignment, uptr HeadersSize, uptr &EntryHeaderPos)
+ EXCLUDES(Mutex) {
const uptr PageSize = getPageSizeCached();
// 10% of the requested size proved to be the optimal choice for
// retrieving cached blocks after testing several options.
constexpr u32 FragmentedBytesDivisor = 10;
- bool Found = false;
+ bool FoundOptimalFit = false;
CachedBlock Entry;
EntryHeaderPos = 0;
{
@@ -348,47 +358,90 @@ class MapAllocatorCache {
CallsToRetrieve++;
if (EntriesCount == 0)
return {};
- u32 OptimalFitIndex = 0;
+ u16 RetrievedIndex = CachedBlock::InvalidEntry;
uptr MinDiff = UINTPTR_MAX;
- for (u32 I = LRUHead; I != CachedBlock::InvalidEntry;
+
+ // Since allocation sizes don't always match cached memory chunk sizes
+ // we allow some memory to be unused (called fragmented bytes). The
+ // amount of unused bytes is exactly EntryHeaderPos - CommitBase.
+ //
+ // CommitBase CommitBase + CommitSize
+ // V V
+ // +---+------------+-----------------+---+
+ // | | | | |
+ // +---+------------+-----------------+---+
+ // ^ ^ ^
+ // Guard EntryHeaderPos Guard-page-end
+ // page-begin
+ //
+ // [EntryHeaderPos, CommitBase + CommitSize) contains the user data as
+ // well as the header metadata. If EntryHeaderPos - CommitBase exceeds
+ // MaxAllowedFragmentedBytes, the cached memory chunk is not considered
+ // valid for retrieval.
+ for (u16 I = LRUHead; I != CachedBlock::InvalidEntry;
I = Entries[I].Next) {
const uptr CommitBase = Entries[I].CommitBase;
const uptr CommitSize = Entries[I].CommitSize;
const uptr AllocPos =
roundDown(CommitBase + CommitSize - Size, Alignment);
const uptr HeaderPos = AllocPos - HeadersSize;
- if (HeaderPos > CommitBase + CommitSize)
+ if (HeaderPos > CommitBase + CommitSize || HeaderPos < CommitBase)
continue;
- if (HeaderPos < CommitBase ||
- AllocPos > CommitBase + PageSize * MaxUnusedCachePages) {
+
+ const uptr Diff = roundDown(HeaderPos, PageSize) - CommitBase;
+
+ if (Diff > MaxAllowedFragmentedBytes || Diff >= MinDiff)
continue;
- }
- Found = true;
- const uptr Diff = HeaderPos - CommitBase;
- // immediately use a cached block if it's size is close enough to the
- // requested size.
- const uptr MaxAllowedFragmentedBytes =
+
+ MinDiff = Diff;
+ RetrievedIndex = I;
+ EntryHeaderPos = HeaderPos;
+
+ const uptr OptimalFitThesholdBytes =
(CommitBase + CommitSize - HeaderPos) / FragmentedBytesDivisor;
- if (Diff <= MaxAllowedFragmentedBytes) {
- OptimalFitIndex = I;
- EntryHeaderPos = HeaderPos;
+ if (Diff <= OptimalFitThesholdBytes) {
+ FoundOptimalFit = true;
break;
}
- // keep track of the smallest cached block
- // that is greater than (AllocSize + HeaderSize)
- if (Diff > MinDiff)
- continue;
- OptimalFitIndex = I;
- MinDiff = Diff;
- EntryHeaderPos = HeaderPos;
}
- if (Found) {
- Entry = Entries[OptimalFitIndex];
- remove(OptimalFitIndex);
+ if (RetrievedIndex != CachedBlock::InvalidEntry) {
+ Entry = Entries[RetrievedIndex];
+ remove(RetrievedIndex);
SuccessfulRetrieves++;
}
}
+ // The difference between the retrieved memory chunk and the request
+ // size is at most MaxAllowedFragmentedBytes
+ //
+ // / MaxAllowedFragmentedBytes \
+ // +--------------------------+-----------+
+ // | | |
+ // +--------------------------+-----------+
+ // \ Bytes to be released / ^
+ // |
+ // (may or may not be commited)
+ //
+ // The maximum number of bytes released to the OS is capped by
+ // MaxCachePagesToRelease
+ //
+ // TODO : Consider making MaxCachePagesToRelease configurable since
+ // the release to OS API can vary across systems.
+ if (!FoundOptimalFit && Entry.Time != 0) {
+ const uptr FragmentedBytes =
+ roundDown(EntryHeaderPos, PageSize) - Entry.CommitBase;
+ const uptr MaxUnusedCacheBytes = MaxUnusedCachePages * PageSize;
+ if (FragmentedBytes > MaxUnusedCacheBytes) {
+ const uptr MaxCacheBytesToRelease =
+ CachedBlock::MaxCachePagesToRelease * PageSize;
+ uptr BytesToRelease =
+ roundUp(Min<uptr>(MaxCacheBytesToRelease,
+ FragmentedBytes - MaxUnusedCacheBytes),
+ PageSize);
+ Entry.MemMap.releaseAndZeroPagesToOS(Entry.CommitBase, BytesToRelease);
+ }
+ }
+
return Entry;
}
@@ -659,8 +712,20 @@ MapAllocator<Config>::tryAllocateFromCache(const Options &Options, uptr Size,
FillContentsMode FillContents) {
CachedBlock Entry;
uptr EntryHeaderPos;
+ uptr MaxAllowedFragmentedBytes;
+ const uptr PageSize = getPageSizeCached();
+
+ if (LIKELY(!useMemoryTagging<Config>(Options))) {
+ const uptr MaxUnusedCacheBytes = MaxUnusedCachePages * PageSize;
+ const uptr MaxCacheBytesToRelease =
+ CachedBlock::MaxCachePagesToRelease * PageSize;
+ MaxAllowedFragmentedBytes = MaxUnusedCacheBytes + MaxCacheBytesToRelease;
+ } else {
+ MaxAllowedFragmentedBytes = MaxUnusedCachePages * PageSize;
+ }
- Entry = Cache.retrieve(Size, Alignment, getHeadersSize(), EntryHeaderPos);
+ Entry = Cache.retrieve(MaxAllowedFragmentedBytes, Size, Alignment,
+ getHeadersSize(), EntryHeaderPos);
if (!Entry.isValid())
return nullptr;
diff --git a/compiler-rt/lib/scudo/standalone/tests/secondary_test.cpp b/compiler-rt/lib/scudo/standalone/tests/secondary_test.cpp
index e85b6abdb36d22..1f4cab26550cce 100644
--- a/compiler-rt/lib/scudo/standalone/tests/secondary_test.cpp
+++ b/compiler-rt/lib/scudo/standalone/tests/secondary_test.cpp
@@ -281,8 +281,8 @@ struct MapAllocatorCacheTest : public Test {
std::unique_ptr<CacheT> Cache = std::make_unique<CacheT>();
const scudo::uptr PageSize = scudo::getPageSizeCached();
- // The current test allocation size is set to the minimum size
- // needed for the scudo allocator to fall back to the secondary allocator
+ // The current test allocation size is set to the maximum
+ // cache entry size
static constexpr scudo::uptr TestAllocSize =
CacheConfig::getDefaultMaxEntrySize();
@@ -327,7 +327,8 @@ TEST_F(MapAllocatorCacheTest, CacheOrder) {
for (scudo::uptr I = CacheConfig::getEntriesArraySize(); I > 0; I--) {
scudo::uptr EntryHeaderPos;
scudo::CachedBlock Entry =
- Cache->retrieve(TestAllocSize, PageSize, 0, EntryHeaderPos);
+ Cache->retrieve(scudo::MaxUnusedCachePages * PageSize, TestAllocSize,
+ PageSize, 0, EntryHeaderPos);
EXPECT_EQ(Entry.MemMap.getBase(), MemMaps[I - 1].getBase());
}
@@ -336,6 +337,34 @@ TEST_F(MapAllocatorCacheTest, CacheOrder) {
MemMap.unmap();
}
+TEST_F(MapAllocatorCacheTest, PartialChunkHeuristicRetrievalTest) {
+ const scudo::uptr MaxUnusedCacheBytes = PageSize;
+ const scudo::uptr MaxCacheBytesToRelease =
+ scudo::CachedBlock::MaxCachePagesToRelease * PageSize;
+ const scudo::uptr FragmentedBytes =
+ MaxUnusedCacheBytes + MaxCacheBytesToRelease;
+
+ scudo::uptr EntryHeaderPos;
+ scudo::CachedBlock Entry;
+ scudo::MemMapT MemMap = allocate(PageSize + FragmentedBytes);
+ Cache->store(Options, MemMap.getBase(), MemMap.getCapacity(),
+ MemMap.getBase(), MemMap);
+
+ // FragmentedBytes > MaxAllowedFragmentedBytes so PageSize
+ // cannot be retrieved from the cache
+ Entry = Cache->retrieve(/*MaxAllowedFragmentedBytes=*/0, PageSize, PageSize,
+ 0, EntryHeaderPos);
+ EXPECT_FALSE(Entry.isValid());
+
+ // FragmentedBytes == MaxAllowedFragmentedBytes so PageSize
+ // can be retrieved from the cache
+ Entry =
+ Cache->retrieve(FragmentedBytes, PageSize, PageSize, 0, EntryHeaderPos);
+ EXPECT_TRUE(Entry.isValid());
+
+ MemMap.unmap();
+}
+
TEST_F(MapAllocatorCacheTest, MemoryLeakTest) {
std::vector<scudo::MemMapT> MemMaps;
// Fill the cache above MaxEntriesCount to force an eviction
@@ -351,7 +380,8 @@ TEST_F(MapAllocatorCacheTest, MemoryLeakTest) {
for (scudo::uptr I = CacheConfig::getDefaultMaxEntriesCount(); I > 0; I--) {
scudo::uptr EntryHeaderPos;
RetrievedEntries.push_back(
- Cache->retrieve(TestAllocSize, PageSize, 0, EntryHeaderPos));
+ Cache->retrieve(scudo::MaxUnusedCachePages * PageSize, TestAllocSize,
+ PageSize, 0, EntryHeaderPos));
EXPECT_EQ(MemMaps[I].getBase(), RetrievedEntries.back().MemMap.getBase());
}
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