linux-kernel - Re: [PATCH] Adding multiple workers to the loop device.

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Message-ID: <CAByjrT8xiL0BLo1MoidZ5O9ooWZnQdd9TQMgcfUFPxaLOMAFhQ@mail.gmail.com>
Date:   Thu, 23 Jan 2020 11:45:57 -0800
From:   Muraliraja Muniraju <muraliraja.muniraju@...rik.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Adding multiple workers to the loop device.

On Thu, Jan 23, 2020 at 11:29 AM Jens Axboe <axboe@...nel.dk> wrote:
>
>
> Please don't top post, we just lost all context here unless I had fixed
> it up for you.
>
>
> On 1/23/20 12:25 PM, Muraliraja Muniraju wrote:
> >
> > On Thu, Jan 23, 2020 at 10:59 AM Jens Axboe <axboe@...nel.dk> wrote:
> >>
> >> On 1/21/20 12:25 PM, muraliraja.muniraju wrote:
> >>> Current loop device implementation has a single kthread worker and
> >>> drains one request at a time to completion. If the underneath device is
> >>> slow then this reduces the concurrency significantly. To help in these
> >>> cases, adding multiple loop workers increases the concurrency. Also to
> >>> retain the old behaviour the default number of loop workers is 1 and can
> >>> be tuned via the ioctl.
> >>
> >> Have you considered using blk-mq for this? Right now loop just does
> >> some basic checks and then queues for a thread. If you bump nr_hw_queues
> >> up (provide a parameter for that) and set BLK_MQ_F_BLOCKING in the
> >> tag flags, then that might be a more viable approach for handling this.
> >
> > I see that the kernel is already is using the multi queues with the
> > number of hardware queues is 1. But the problem IMO is that the worker
> > seems to be processing 1 request at a time, to parallelize requests
> > and have more concurrency more workers needs to be added. I also tried
> > increasing the nr_hw_queues without increasing the number of workers,
> > I did not see any difference in performance and it stayed the same. It
> > allows to queue more requests but it is processed one at a time. I
> > have not tried with enabling BLK_MQ_F_BLOCKING though. I see that it
> > can schedule requests early.
>
> The experiment is useless without BLK_MQ_F_BLOCKING set, so you need
> that at least. With that, you _will_ see work items processed in
> parallel, depending on where they are queued from.
>
> --
> Jens Axboe
>
Sure, let me try setting the BLK_MQ_F_BLOCKING on the existing patch
that I sent and see. Will update soon, Thanks.