linux-kernel - Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only)

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Date:	Mon, 30 Mar 2015 13:37:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Allison <jra@...ba.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Milosz Tanski <milosz@...in.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>, Al Viro <viro@...iv.linux.org.uk>,
	linux-api@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-arch@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache
 only)

On Mon, 30 Mar 2015 13:32:27 -0700 Jeremy Allison <jra@...ba.org> wrote:

> On Mon, Mar 30, 2015 at 01:26:25PM -0700, Andrew Morton wrote:
> > 
> > cons:
> > 
> > d) fincore() is more expensive
> > 
> > e) fincore() will very occasionally block
> 
> The above is the killer for Samba. If fincore
> returns true but when we schedule the pread
> we block, we're hosed.
> 
> Once we block, we're done serving clients on the main
> thread until this returns. That can cause unpredictable
> response times which can cause client timeouts.
> 
> A fincore+pread solution that blocks is simply unsafe
> to use for us. We'll have to stay with the threadpool :-(.

Finally.  Thanks ;)

This implies that the samba main thread also has to avoid any memory
allocations both direct and within syscall and pagefault - those will
occasionally exhibit similar worse-case latency. Is this done now?


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