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fix advance reservation of arena block addresses 

Summary:
Calling `std::vector::reserve()` causes memory to be reallocated and then data to be moved. It was called prior to adding every block. This reallocation could be done a huge amount of times, e.g., for users with large index blocks.

Instead, we can simply use `std::vector::emplace_back()` in such a way that preserves the no-memory-leak guarantee, while letting the vector decide when to reallocate space. Now I see reallocation/moving happen O(logN) times, rather than O(N) times, where N is the final size of vector.
Closes https://github.com/facebook/rocksdb/pull/3508

Differential Revision: D6994228

Pulled By: ajkr

fbshipit-source-id: ab7c11e13ff37c8c6c8249be7a79566a4068cd27
This commit is contained in:
Andrew Kryczka 2018-02-15 19:30:52 -08:00 committed by Facebook Github Bot
parent 989d12313c
commit 0454f781c2
1 changed files with 23 additions and 14 deletions
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37
util/arena.cc
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@ -79,6 +79,9 @@ Arena::~Arena() {
#ifdef MAP_HUGETLB
for (const auto& mmap_info : huge_blocks_) {
if (mmap_info.addr_ == nullptr) {
continue;
}
auto ret = munmap(mmap_info.addr_, mmap_info.length_);
if (ret != 0) {
// TODO(sdong): Better handling
@ -126,11 +129,15 @@ char* Arena::AllocateFromHugePage(size_t bytes) {
if (hugetlb_size_ == 0) {
return nullptr;
}
// already reserve space in huge_blocks_ before calling mmap().
// this way the insertion into the vector below will not throw and we
// won't leak the mapping in that case. if reserve() throws, we
// won't leak either
huge_blocks_.reserve(huge_blocks_.size() + 1);
// Reserve space in `huge_blocks_` before calling `mmap`.
// Use `emplace_back()` instead of `reserve()` to let std::vector manage its
// own memory and do fewer reallocations.
//
// - If `emplace_back` throws, no memory leaks because we haven't called
// `mmap` yet.
// - If `mmap` throws, no memory leaks because the vector will be cleaned up
// via RAII.
huge_blocks_.emplace_back(nullptr /* addr */, 0 /* length */);
void* addr = mmap(nullptr, bytes, (PROT_READ | PROT_WRITE),
(MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB), -1, 0);
@ -138,8 +145,7 @@ char* Arena::AllocateFromHugePage(size_t bytes) {
if (addr == MAP_FAILED) {
return nullptr;
}
// the following shouldn't throw because of the above reserve()
huge_blocks_.emplace_back(MmapInfo(addr, bytes));
huge_blocks_.back() = MmapInfo(addr, bytes);
blocks_memory_ += bytes;
if (tracker_ != nullptr) {
tracker_->Allocate(bytes);
@ -193,11 +199,15 @@ char* Arena::AllocateAligned(size_t bytes, size_t huge_page_size,
}
char* Arena::AllocateNewBlock(size_t block_bytes) {
// already reserve space in blocks_ before allocating memory via new.
// this way the insertion into the vector below will not throw and we
// won't leak the allocated memory in that case. if reserve() throws,
// we won't leak either
blocks_.reserve(blocks_.size() + 1);
// Reserve space in `blocks_` before allocating memory via new.
// Use `emplace_back()` instead of `reserve()` to let std::vector manage its
// own memory and do fewer reallocations.
//
// - If `emplace_back` throws, no memory leaks because we haven't called `new`
// yet.
// - If `new` throws, no memory leaks because the vector will be cleaned up
// via RAII.
blocks_.emplace_back(nullptr);
char* block = new char[block_bytes];
size_t allocated_size;
@ -216,8 +226,7 @@ char* Arena::AllocateNewBlock(size_t block_bytes) {
if (tracker_ != nullptr) {
tracker_->Allocate(allocated_size);
}
// the following shouldn't throw because of the above reserve()
blocks_.push_back(block);
blocks_.back() = block;
return block;
}