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

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Date:   Fri, 5 Jun 2020 16:49:24 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Andres Freund <andres@...razel.de>
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

On 6/5/20 4:36 PM, Andres Freund wrote:
> Hi,
> 
> On 2020-06-05 15:30:44 -0700, Andres Freund wrote:
>> On 2020-06-05 15:21:34 -0600, Jens Axboe wrote:
>>>>> I can reproduce this, and I see what it is. I'll send out a patch soonish.
>>>>
>>>> Thinko, can you try with this on top?
>>>
>>> Sorry that was incomplete, please use this one!
>>
>> That seems to fix it! Yay.
>>
>>
>> Bulk buffered reads somehow don't quite seem to be performing that well
>> though, looking into it. Could be on the pg side too.
> 
> While looking into that, I played with setting
> /sys/<dev>/queue/read_ahead_kb to 0 and noticed that seems to result in
> all/most IO done in workers. Is that to be expected?

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.

-- 
Jens Axboe