Use best-fit strategy in Arena, now O(log(n)) instead O(n)

This replaces the first-fit algorithm used in the Arena with a best-fit. According to "Dynamic Storage Allocation: A Survey and Critical Review", Wilson et. al. 1995, http://www.scs.stanford.edu/14wi-cs140/sched/readings/wilson.pdf, both startegies work well in practice.

The advantage of using best-fit is that we can switch the slow O(n) algorithm to O(log(n)) operations. Additionally, some previously O(log(n)) operations are now replaced with O(1) operations by using a hash map. The end effect is that the benchmark runs about 2.5 times faster on my machine:

old: BenchLockedPool, 5, 530, 5.25749, 0.00196938, 0.00199755, 0.00198172
new: BenchLockedPool, 5, 1300, 5.11313, 0.000781493, 0.000793314, 0.00078606

I've run all unit tests and benchmarks.
This commit is contained in:
Martin Ankerl 2017-12-29 11:36:11 +01:00
parent 5180a86c96
commit 1e0ee9095c
3 changed files with 60 additions and 31 deletions

View file

@ -43,4 +43,4 @@ static void BenchLockedPool(benchmark::State& state)
addr.clear();
}
BENCHMARK(BenchLockedPool, 530);
BENCHMARK(BenchLockedPool, 1300);

View file

@ -47,7 +47,9 @@ Arena::Arena(void *base_in, size_t size_in, size_t alignment_in):
base(static_cast<char*>(base_in)), end(static_cast<char*>(base_in) + size_in), alignment(alignment_in)
{
// Start with one free chunk that covers the entire arena
chunks_free.emplace(base, size_in);
auto it = size_to_free_chunk.emplace(size_in, base);
chunks_free.emplace(base, it);
chunks_free_end.emplace(base + size_in, it);
}
Arena::~Arena()
@ -63,26 +65,30 @@ void* Arena::alloc(size_t size)
if (size == 0)
return nullptr;
// Pick a large enough free-chunk
auto it = std::find_if(chunks_free.begin(), chunks_free.end(),
[=](const std::map<char*, size_t>::value_type& chunk){ return chunk.second >= size; });
if (it == chunks_free.end())
// Pick a large enough free-chunk. Returns an iterator pointing to the first element that is not less than key.
// This allocation strategy is best-fit. According to "Dynamic Storage Allocation: A Survey and Critical Review",
// Wilson et. al. 1995, http://www.scs.stanford.edu/14wi-cs140/sched/readings/wilson.pdf, best-fit and first-fit
// policies seem to work well in practice.
auto sizePtrIt = size_to_free_chunk.lower_bound(size);
if (sizePtrIt == size_to_free_chunk.end())
return nullptr;
// Create the used-chunk, taking its space from the end of the free-chunk
auto alloced = chunks_used.emplace(it->first + it->second - size, size).first;
if (!(it->second -= size))
chunks_free.erase(it);
return reinterpret_cast<void*>(alloced->first);
const size_t sizeRemaining = sizePtrIt->first - size;
auto alloced = chunks_used.emplace(sizePtrIt->second + sizeRemaining, size).first;
chunks_free_end.erase(sizePtrIt->second + sizePtrIt->first);
if (sizePtrIt->first == size) {
// whole chunk is used up
chunks_free.erase(sizePtrIt->second);
} else {
// still some memory left in the chunk
auto itRemaining = size_to_free_chunk.emplace(sizeRemaining, sizePtrIt->second);
chunks_free[sizePtrIt->second] = itRemaining;
chunks_free_end.emplace(sizePtrIt->second + sizeRemaining, itRemaining);
}
size_to_free_chunk.erase(sizePtrIt);
/* extend the Iterator if other begins at its end */
template <class Iterator, class Pair> bool extend(Iterator it, const Pair& other) {
if (it->first + it->second == other.first) {
it->second += other.second;
return true;
}
return false;
return reinterpret_cast<void*>(alloced->first);
}
void Arena::free(void *ptr)
@ -97,25 +103,39 @@ void Arena::free(void *ptr)
if (i == chunks_used.end()) {
throw std::runtime_error("Arena: invalid or double free");
}
auto freed = *i;
std::pair<char*, size_t> freed = *i;
chunks_used.erase(i);
// Add space to free map, coalescing contiguous chunks
auto next = chunks_free.upper_bound(freed.first);
auto prev = (next == chunks_free.begin()) ? chunks_free.end() : std::prev(next);
if (prev == chunks_free.end() || !extend(prev, freed))
prev = chunks_free.emplace_hint(next, freed);
if (next != chunks_free.end() && extend(prev, *next))
// Coalesc freed with previous chunk
auto prev = chunks_free_end.find(freed.first);
if (prev != chunks_free_end.end()) {
freed.first -= prev->second->first;
freed.second += prev->second->first;
size_to_free_chunk.erase(prev->second);
chunks_free_end.erase(prev);
}
// Coalesc freed with chunk after freed
auto next = chunks_free.find(freed.first + freed.second);
if (next != chunks_free.end()) {
freed.second += next->second->first;
size_to_free_chunk.erase(next->second);
chunks_free.erase(next);
}
// Add/set space with coalesced free chunk
auto it = size_to_free_chunk.emplace(freed.second, freed.first);
chunks_free[freed.first] = it;
chunks_free_end[freed.first + freed.second] = it;
}
Arena::Stats Arena::stats() const
{
Arena::Stats r{ 0, 0, 0, chunks_used.size(), chunks_free.size() };
for (const auto& chunk: chunks_used)
r.used += chunk.second;
for (const auto& chunk: chunks_free)
r.free += chunk.second;
r.free += chunk.second->first;
r.total = r.used + r.free;
return r;
}

View file

@ -10,6 +10,7 @@
#include <map>
#include <mutex>
#include <memory>
#include <unordered_map>
/**
* OS-dependent allocation and deallocation of locked/pinned memory pages.
@ -88,11 +89,19 @@ public:
*/
bool addressInArena(void *ptr) const { return ptr >= base && ptr < end; }
private:
/** Map of chunk address to chunk information. This class makes use of the
* sorted order to merge previous and next chunks during deallocation.
*/
std::map<char*, size_t> chunks_free;
std::map<char*, size_t> chunks_used;
typedef std::multimap<size_t, char*> SizeToChunkSortedMap;
/** Map to enable O(log(n)) best-fit allocation, as it's sorted by size */
SizeToChunkSortedMap size_to_free_chunk;
typedef std::unordered_map<char*, SizeToChunkSortedMap::const_iterator> ChunkToSizeMap;
/** Map from begin of free chunk to its node in size_to_free_chunk */
ChunkToSizeMap chunks_free;
/** Map from end of free chunk to its node in size_to_free_chunk */
ChunkToSizeMap chunks_free_end;
/** Map from begin of used chunk to its size */
std::unordered_map<char*, size_t> chunks_used;
/** Base address of arena */
char* base;
/** End address of arena */