linux-kernel - Re: fadvise interferes with readahead

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Message-ID: <CAGTBQpY-yav5G4aPSBdUmACWQbe8RR=8OnRwKHbMuuR=GBgBxw@mail.gmail.com>
Date:	Tue, 20 Nov 2012 12:05:02 -0300
From:	Claudio Freire <klaussfreire@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	linux-kernel@...r.kernel.org,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: fadvise interferes with readahead

On Tue, Nov 20, 2012 at 11:58 AM, Fengguang Wu <fengguang.wu@...el.com> wrote:
>
>> But if cache hits were to simply update
>> readahead state, it would only mean that read calls behave the same
>> regardless of fadvise calls. I think that's worth pursuing.
>
> Here you are describing an alternative solution that will somehow trap
> into the readahead code even when, for example, the application is
> accessing once and again an already cached file?  I'm afraid this will
> add non-trivial overheads and is less attractive than the "readahead
> on fadvise" solution.

Not for all cache hits, only those in state !PageUptodate, which are
I/O in progress, the case that hurts.

>> I ought to try to prepare a patch for this to illustrate my point. Not
>> sure I'll be able to though.
>
> I'd be glad to materialize the readahead on fadvise proposal, if there
> are no obvious negative examples/cases.

I don't expect a significant performance hit if only !PageUptodate
hits invoke readahead code. But I'm no kernel expert either.
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