linux-kernel - Re: missing madvise functionality

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Message-ID: <461479B8.9090203@yahoo.com.au>
Date:	Thu, 05 Apr 2007 14:23:20 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jakub Jelinek <jakub@...hat.com>
CC:	Ulrich Drepper <drepper@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Memory Management <linux-mm@...ck.org>
Subject: Re: missing madvise functionality

Nick Piggin wrote:
> Jakub Jelinek wrote:
> 
>> On Wed, Apr 04, 2007 at 05:46:12PM +1000, Nick Piggin wrote:
>>
>>> Does mmap(PROT_NONE) actually free the memory?
>>
>>
>>
>> Yes.
>>         /* Clear old maps */
>>         error = -ENOMEM;
>> munmap_back:
>>         vma = find_vma_prepare(mm, addr, &prev, &rb_link, &rb_parent);
>>         if (vma && vma->vm_start < addr + len) {
>>                 if (do_munmap(mm, addr, len))
>>                         return -ENOMEM;
>>                 goto munmap_back;
>>         }
> 
> 
> Thanks, I overlooked the mmap vs mprotect detail. So how are the subsequent
> access faults avoided?

AFAIKS, the faults are not avoided. Not for single page allocations, not
for multi-page allocations.

So what glibc currently does to allocate, use, then deallocate a page is
this:
   mprotect(PROT_READ|PROT_WRITE) -> down_write(mmap_sem)
   touch page -> page fault -> down_read(mmap_sem)
   mmap(PROT_NONE) -> down_write(mmap_sem)

What it could be doing is:
   touch page -> page fault -> down_read(mmap_sem)
   madvise(MADV_DONTNEED) -> down_read(mmap_sem)

So after my previously posted patch (attached again) to only take down_read
in madvise where possible...

With 2 threads, the attached test.c ends up doing about 140,000 context
switches per second with just 2 threads/2CPUs, takes a little over 2
million faults, and about 80 seconds to complete, when running the
old_test() function (ie. mprotect,touch,mmap).

When running new_test() (ie. touch,madvise), context switches stay well
under 100, it takes slightly fewer faults, and it completes in about 8
seconds.

With 1 thread, new_test() actually completes in under half the time as
well (4.55 vs 9.88 seconds). This result won't have been altered by my
madvise patch, because the down_write fastpath is no slower than down_read.

Any comments?

-- 
SUSE Labs, Novell Inc.

View attachment "madv-mmap_sem.patch" of type "text/plain" (1305 bytes)

View attachment "test.c" of type "text/x-csrc" (1869 bytes)