forked from OSchip/llvm-project
0dd40cf28d
Summary: Currently, AllocateRegion has a tendency to fragment memory: it allocates `2*kRegionSize`, and if the memory is aligned, will unmap `kRegionSize` bytes, thus creating a hole, which can't itself be reused for another region. This is exacerbated by the fact that if 2 regions get allocated one after another without any `mmap` in between, the second will be aligned due to mappings generally being contiguous. An idea, suggested by @alekseyshl, to prevent such a behavior is to have a stash of regions: if the `2*kRegionSize` allocation is properly aligned, split it in two, and stash the second part to be returned next time a region is requested. At this point, I thought about a couple of ways to implement this: - either an `IntrusiveList` of regions candidates, storing `next` at the begining of the region; - a small array of regions candidates existing in the Primary. While the second option is more constrained in terms of size, it offers several advantages: - security wise, a pointer in a region candidate could be overflowed into, and abused when popping an element; - we do not dirty the first page of the region by storing something in it; - unless several threads request regions simultaneously from different size classes, the stash rarely goes above 1 entry. I am not certain about the Windows impact of this change, as `sanitizer_win.cc` has its own version of MmapAlignedOrDie, maybe someone could chime in on this. MmapAlignedOrDie is effectively unused after this change and could be removed at a later point. I didn't notice any sizeable performance gain, even though we are saving a few `mmap`/`munmap` syscalls. Reviewers: alekseyshl, kcc, dvyukov Reviewed By: alekseyshl Subscribers: llvm-commits, kubamracek Differential Revision: https://reviews.llvm.org/D33454 llvm-svn: 303879 |
||
|---|---|---|
| clang | ||
| clang-tools-extra | ||
| compiler-rt | ||
| debuginfo-tests | ||
| libclc | ||
| libcxx | ||
| libcxxabi | ||
| libunwind | ||
| lld | ||
| lldb | ||
| llgo | ||
| llvm | ||
| openmp | ||
| parallel-libs | ||
| polly | ||