linux-kernel - Re: [PATCH] MM: implement MADV_FREE lazy freeing of anonymous memory

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Date:	Tue, 8 May 2007 14:35:28 -0400
From:	Jakub Jelinek <jakub@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Rik van Riel <riel@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] MM: implement MADV_FREE lazy freeing of anonymous memory

On Tue, May 08, 2007 at 04:12:00PM +1000, Nick Piggin wrote:
> I didn't actually check system and user times for the mysql
> benchmark, but that's exactly what I had in mind when I
> mentioned the poor cache behaviour this patch could cause. I
> definitely did see user times go up in benchmarks where I
> measured.
> 
> We have percpu and cache affine page allocators, so when
> userspace just frees a page, it is likely to be cache hot, so
> we want to free it up so it can be reused by this CPU ASAP.
> Likewise, when we newly allocate a page, we want it to be one
> that is cache hot on this CPU.

malloc has per-thread arenas, so when using MADV_FREE the pages
should be local to the thread as well (unless the thread has switched
to a different CPU also to the CPU) and in case of sysbench should
be cache hot as well (it is reused RSN).  With MADV_DONTNEED you need to
clear the pages while that is not necessary with MADV_FREE.

	Jakub
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