linux-kernel - Re: Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

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Message-ID: <0598fe2754bf0717d81f7e72d3e9b3230c608cc6.camel@unipv.it>
Date:   Tue, 26 Nov 2019 08:46:07 +0100
From:   Andrea Vai <andrea.vai@...pv.it>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Damien Le Moal <Damien.LeMoal@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Jens Axboe <axboe@...nel.dk>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        USB list <linux-usb@...r.kernel.org>,
        SCSI development list <linux-scsi@...r.kernel.org>,
        Himanshu Madhani <himanshu.madhani@...ium.com>,
        Hannes Reinecke <hare@...e.com>,
        Omar Sandoval <osandov@...com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Hans Holmberg <Hans.Holmberg@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Slow I/O on USB media after commit
 f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

Il giorno mar, 26/11/2019 alle 10.32 +0800, Ming Lei ha scritto:
> On Mon, Nov 25, 2019 at 07:51:33PM +0100, Andrea Vai wrote:
> > Il giorno lun, 25/11/2019 alle 23.15 +0800, Ming Lei ha scritto:
> > > On Mon, Nov 25, 2019 at 03:58:34PM +0100, Andrea Vai wrote:
> > > 
> > > [...]
> > > 
> > > > What to try next?
> > > 
> > > 1) cat /sys/kernel/debug/block/$DISK/hctx0/flags
> > result:
> > 
> > alloc_policy=FIFO SHOULD_MERGE|2
> > 
> > > 
> > > 
> > > 2) echo 128 > /sys/block/$DISK/queue/nr_requests and run your
> copy
> > > 1GB
> > > test again.
> > 
> > done, and still fails. What to try next?
> 
> I just run 256M cp test

I would like to point out that 256MB is a filesize that usually don't
trigger the issue (don't know if it matters, sorry).

Another info I would provide is about another strange behavior I
noticed: yesterday I ran the test two times (as usual with 1GB
filesize) and took 2370s, 1786s, and a third test was going on when I
stopped it. Then I started another set of 100 trials and let them run
tonight, and the first 10 trials were around 1000s, then gradually
decreased to ~300s, and finally settled around 200s with some trials
below 70-80s. This to say, times are extremely variable and for the
first time I noticed a sort of "performance increase" with time.

>  to one USB storage device on patched kernel,
> and WRITE data IO is really in ascending order. The filesystem is
> ext4,
> and mount without '-o sync'. From previous discussion, looks that is
> exactly your test setting. The order can be observed via the
> following script:
> 
> #!/bin/sh
> MAJ=$1
> MIN=$2
> MAJ=$(( $MAJ << 20 ))
> DEV=$(( $MAJ | $MIN ))
> /usr/share/bcc/tools/trace -t -C \
>   't:block:block_rq_issue (args->dev == '$DEV') "%s %d %d", args-
> >rwbs, args->sector, args->nr_sector'
> 
> $MAJ & $MIN can be retrieved via lsblk for your USB storage disk.
> 
> So I think we need to check if the patch is applied correctly first.
> 
> If your kernel tree is managed via git,
yes it is,

>  please post 'git diff'.
attached. Is it correctly patched? thanks.

> Otherwise, share us your kernel version,
btw, is 5.4.0+

>  and I will send you one
> backported patch on the kernel version.
> 
> Meantime, you can collect IO order log via the above script as you
> did last
> time, then send us the log.

ok, will try; is it just required to run it for a short period of time
(say, some seconds) during the copy, or should I run it before the
beginning (or before the mount?), and terminate it after the end of
the copy? (Please note that in the latter case a large amount of time
(and data, I suppose) would be involved, because, as said, to be sure
the problem triggers I have to use a large file... but we can try to
better understand and tune this. If it can help, you can get an ods
file with the complete statistic at [1] (look at the "prove_nov19"
sheet)).

Thanks,
Andrea

[1]: http://fisica.unipv.it/transfer/kernelstats.zip

View attachment "git_diff.txt" of type "text/plain" (6424 bytes)