On Wed, Nov 04, 2015 at 05:05:47PM -0500, Daniel Micay wrote: > > With enough pages at once, though, munmap would be fine, too. > > That implies lots of page faults and zeroing though. The zeroing alone > is a major performance issue. > > There are separate issues with munmap since it ends up resulting in a > lot more virtual memory fragmentation. It would help if the kernel used > first-best-fit for mmap instead of the current naive algorithm (bonus: > O(log n) worst-case, not O(n)). Since allocators like jemalloc and > PartitionAlloc want 2M aligned spans, mixing them with other allocators > can also accelerate the VM fragmentation caused by the dumb mmap > algorithm (i.e. they make a 2M aligned mapping, some other mmap user > does 4k, now there's a nearly 2M gap when the next 2M region is made and > the kernel keeps going rather than reusing it). Anyway, that's a totally > separate issue from this. Just felt like complaining :). > > > Maybe what's really needed is a MADV_FREE variant that takes an iovec. > > On an all-cores multithreaded mm, the TLB shootdown broadcast takes > > thousands of cycles on each core more or less regardless of how much > > of the TLB gets zapped. > > That would work very well. The allocator ends up having a sequence of > dirty spans that it needs to purge in one go. As long as purging is > fairly spread out, the cost of a single TLB shootdown isn't that bad. It > is extremely bad if it needs to do it over and over to purge a bunch of > ranges, which can happen if the memory has ended up being very, very > fragmentated despite the efforts to compact it (depends on what the > application ends up doing).
I posted a patch doing exactly iovec madvise. Doesn't support MADV_FREE yet though, but should be easy to do it. http://marc.info/?l=linux-mm&m=144615663522661&w=2 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
