Large allocations (>= 32 Mb) ignore reserved OS memory #1115
Description
Hi. Thank you for this excellent project.
We noticed that large allocations are not served from preallocated arenas in mimalloc 3 (at least, in 3.0.3).
Consider a simple repro example:
$ cat repro.cpp
#include <vector>
__attribute__((noinline)) std::vector<char> CreateLargeBuffer() {
std::vector<char> buffer(1024ull * 1024 * 32); // Allocate 32 Mb from heap.
return buffer;
}
int main() {
for (int i = 0; i < 100; ++i) { // Allocate memory 100 times.
auto buffer = CreateLargeBuffer();
(void)buffer; // Prevent unused variable warning.
}
return 0;
}
# Trace mmap call stacks for the program run.
$ MIMALLOC_RESERVE_OS_MEMORY=10GB strace -k -f -e mmap,munmap -o log ./repro
$ grep 'CreateLarge' log | wc -l
200 # 100 mmap and munmap pairs.
We see that despite the 10 GB arena preallocated with MIMALLOC_RESERVE_OS_MEMORY=10GB every allocation call still ends up in mmap and munmap call pair. This was quite unexpected.
Arena preallocation was recommended in another issue: #447. But it looks like this workaround doesn't work anymore.
Does someone have any suggestion about preventing mapping/unmapping memory? Is it a regression or somewhat expected?