linux-kernel - Re: [RFC v1][PATCH]page_fault retry with NOPAGE

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Date:	Fri, 21 Nov 2008 23:15:11 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ying Han <yinghan@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Mike Waychison <mikew@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Rohit Seth <rohitseth@...gle.com>
Subject: Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY

On Fri, 21 Nov 2008 22:47:44 -0800 Ying Han <yinghan@...gle.com> wrote:

> page fault retry with NOPAGE_RETRY
> Allow major faults to drop the mmap_sem read lock while waitting for
> synchronous disk read. This allows another thread which wishes to grab
> down_read(mmap_sem) to proceed while the current is waitting the disk IO.

Confused.  down_read() on an rwsem will already permit multiple threads
to run that section of ccode concurrently.

The benefit here will be to permit down_write() callers (eg:
sys_mmap()) to get in there and do work.

> The patch flags current->flags to PF_FAULT_MAYRETRY as identify that the
> caller can tolerate the retry in the filemap_fault call patch.
> 
> Benchmark is done by mmap in huge file and spaw 64 thread each faulting in
> pages in reverse order, the the result shows 8% porformance hit with the
> patch.

You mean it slowed down 8%?  I'm a bit surprised - I'd have expected a
smaller slowdown for an IO-intensive thing like this.

Does it speed anything up?

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