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Date:   Sun, 23 Jul 2023 13:43:50 -0400
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Subject: Re: [PATCH 6.4 800/800] io_uring: Use io_schedule* in cqring wait
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To:     Jens Axboe <axboe@kernel.dk>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        stable@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andres Freund <andres@anarazel.de>
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 <12251678.O9o76ZdvQC@natalenko.name>
 <538065ee-4130-6a00-dcc8-f69fbc7d7ba0@kernel.dk>
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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%)