Re: [PATCH v2 01/13] mm: support madvise(MADV_FREE)

From: Shaohua Li
Date: Thu Nov 05 2015 - 13:17:36 EST

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.
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