linux-kernel - Re: [PATCH 6.4 800/800] io_uring: Use io

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Message-ID: <2691683.mvXUDI8C0e@natalenko.name>
Date:   Sun, 23 Jul 2023 20:06:19 +0200
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Jens Axboe <axboe@...nel.dk>, stable@...r.kernel.org,
        Genes Lists <lists@...ience.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andres Freund <andres@...razel.de>
Subject: Re: [PATCH 6.4 800/800] io_uring: Use io_schedule* in cqring wait

Hello.

On neděle 23. července 2023 19:43:50 CEST Genes Lists wrote:
> On 7/23/23 11:31, Jens Axboe wrote:
> ...
> > Just read the first one, but this is very much expected. It's now just
> > correctly reflecting that one thread is waiting on IO. IO wait being
> > 100% doesn't mean that one core is running 100% of the time, it just
> > means it's WAITING on IO 100% of the time.
> > 
> 
> Seems reasonable thank you.
> 
> Question - do you expect the iowait to stay high for a freshly created 
> mariadb doing nothing (as far as I can tell anyway) until process 
> exited? Or Would you think it would drop in this case prior to the 
> process exiting.
> 
> For example I tried the following - is the output what you expect?
> 
> Create a fresh mariab with no databases - monitor the core showing the 
> iowaits with:
> 
>     mpstat -P ALL 2 100
> 
> # rm -f /var/lib/mysql/*
> # mariadb-install-db --user=mysql --basedir=/usr --datadir=/var/lib/mysql
> 
> # systemctl start mariadb      (iowaits -> 100%) 
>  
> 
> # iotop -bo |grep maria        (shows no output, iowait stays 100%)
> 
> (this persists until mariadb process exits)
>  
> 
> # systemctl stop mariadb       (iowait drops to 0%) 

This is a visible userspace behaviour change with no changes in the userspace itself, so we cannot just ignore it. If for some reason this is how it should be now, how do we explain it to MariaDB devs to get this fixed?

Thanks.

-- 
Oleksandr Natalenko (post-factum)
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