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

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Message-ID: <70e5349a-87af-a2ea-f871-95270f57c6e3@sapience.com>
Date:   Sun, 23 Jul 2023 13:43:50 -0400
From:   Genes Lists <lists@...ience.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        stable@...r.kernel.org
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

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%)