linux-kernel - Re: [PATCHSET v5 0/12] Add support for async buffered reads

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Message-ID: <20200605230244.u342xw4negic2nmr@alap3.anarazel.de>
Date:   Fri, 5 Jun 2020 16:02:44 -0700
From:   Andres Freund <andres@...razel.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCHSET v5 0/12] Add support for async buffered reads

Hi,

On 2020-06-05 16:56:44 -0600, Jens Axboe wrote:
> On 6/5/20 4:54 PM, Andres Freund wrote:
> > On 2020-06-05 16:49:24 -0600, Jens Axboe wrote:
> >> Yes that's expected, if we have to fallback to ->readpage(), then it'll
> >> go to a worker. read-ahead is what drives the async nature of it, as we
> >> issue the range (plus more, depending on RA window) as read-ahead for
> >> the normal read, then wait for it.
> > 
> > But I assume async would still work for files with POSIX_FADV_RANDOM
> > set, or not? Assuming the system wide setting isn't zero, of course.
> 
> Yes it'll work if FADV_RANDOM is set.

Cool.


> But just not if read-ahead is totally disabled. I guess we could make
> that work too, though not sure that it's super important.

It's not from my end. I was just trying to check if the reduced
performance I saw was related to interactions between PG prefetching and
kernel level prefetching. And changing the /sys entry seemed easier than
making postgres set POSIX_FADV_RANDOM...

Greetings,

Andres Freund