From: Ming Lei <ming.lei@redhat.com>
To: Sagi Grimberg <sagi@grimberg.me>
Cc: Christoph Hellwig <hch@infradead.org>,
Baolin Wang <baolin.wang7@gmail.com>,
axboe@kernel.dk, ulf.hansson@linaro.org, adrian.hunter@intel.com,
arnd@arndb.de, linus.walleij@linaro.org,
paolo.valente@linaro.org, orsonzhai@gmail.com,
zhang.lyra@gmail.com, linux-mmc@vger.kernel.org,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 1/7] block: Extand commit_rqs() to do batch processing
Date: Sat, 9 May 2020 07:22:22 +0800 [thread overview]
Message-ID: <20200508232222.GA1391368@T590> (raw)
In-Reply-To: <fe6bd8b9-6ed9-b225-f80c-314746133722@grimberg.me>
Hi Sagi,
On Fri, May 08, 2020 at 03:19:45PM -0700, Sagi Grimberg wrote:
> Hey Ming,
>
> > > Would it make sense to elevate this flag to a request_queue flag
> > > (QUEUE_FLAG_ALWAYS_COMMIT)?
> >
> > request queue flag usually is writable, however this case just needs
> > one read-only flag, so I think it may be better to make it as
> > tagset/hctx flag.
>
> I actually intended it to be writable.
>
> > > I'm thinking of a possibility that an I/O scheduler may be used
> > > to activate this functionality rather than having the driver set
> > > it necessarily...
> >
> > Could you explain a bit why I/O scheduler should activate this
> > functionality?
>
> Sure, I've recently seen some academic work showing the benefits
> of batching in tcp/ip based block drivers. The problem with the
> approaches taken is that I/O scheduling is exercised deep down in the
> driver, which is not the direction I'd like to go if we are want
> to adopt some of the batching concepts.
>
> I spent some (limited) time thinking about this, and it seems to
> me that there is an opportunity to implement this as a dedicated
> I/O scheduler, and tie it to driver specific LLD stack optimizations
> (net-stack for example) relying on the commit_rq/bd->last hints.
>
> When scanning the scheduler code, I noticed exactly the phenomenon that
> this patchset is attempting to solve and Christoph referred me to it.
> Now I'm thinking if we can extend this batching optimization for both
> use-cases.
Got it, thanks for the sharing.
>
> > batching submission may be good for some drivers, and currently
> > we only do it in limited way. One reason is that there is extra
> > cost for full batching submission, such as this patch requires
> > one extra .commit_rqs() for each dispatch, and lock is often needed
> > in this callback.
>
> That is not necessarily the case at all.
So far, all in-tree .commit_rqs() implementation requires lock.
>
> > IMO it can be a win for some slow driver or device, but may cause
> > a little performance drop for fast driver/device especially in workload
> > of not-batching submission.
>
> You're mostly correct. This is exactly why an I/O scheduler may be
> applicable here IMO. Mostly because I/O schedulers tend to optimize for
> something specific and always present tradeoffs. Users need to
> understand what they are optimizing for.
>
> Hence I'd say this functionality can definitely be available to an I/O
> scheduler should one exist.
>
I guess it is just that there can be multiple requests available from
scheduler queue. Actually it can be so for other non-nvme drivers in
case of none, such as SCSI.
Another way is to use one per-task list(such as plug list) to hold the
requests for dispatch, then every drivers may see real .last flag, so they
may get chance for optimizing batch queuing. I will think about the
idea further and see if it is really doable.
Thanks,
Ming
next prev parent reply other threads:[~2020-05-08 23:22 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-26 9:38 [RFC PATCH v2 0/7] Add MMC packed request support Baolin Wang
2020-04-26 9:38 ` [RFC PATCH v2 1/7] block: Extand commit_rqs() to do batch processing Baolin Wang
2020-04-27 15:46 ` Christoph Hellwig
2020-04-28 8:02 ` Baolin Wang
2020-05-08 21:35 ` Sagi Grimberg
2020-05-08 21:46 ` Ming Lei
2020-05-08 22:19 ` Sagi Grimberg
2020-05-08 23:22 ` Ming Lei [this message]
2020-05-09 8:57 ` Baolin Wang
2020-05-09 9:43 ` Ming Lei
2020-05-10 7:44 ` Sagi Grimberg
2020-05-11 1:29 ` Ming Lei
2020-05-11 9:23 ` Sagi Grimberg
2020-05-11 11:47 ` Ming Lei
2020-05-12 6:26 ` Sagi Grimberg
2020-05-12 7:55 ` Ming Lei
2020-04-26 9:38 ` [RFC PATCH v2 2/7] mmc: Add MMC packed request support for MMC software queue Baolin Wang
2020-04-26 9:38 ` [RFC PATCH v2 3/7] mmc: host: sdhci: Introduce ADMA3 transfer mode Baolin Wang
2020-04-26 9:38 ` [RFC PATCH v2 4/7] mmc: host: sdhci: Factor out the command configuration Baolin Wang
2020-04-26 9:38 ` [RFC PATCH v2 5/7] mmc: host: sdhci: Remove redundant sg_count member of struct sdhci_host Baolin Wang
2020-04-26 9:38 ` [RFC PATCH v2 6/7] mmc: host: sdhci: Add MMC packed request support Baolin Wang
2020-04-26 9:39 ` [RFC PATCH v2 7/7] mmc: host: sdhci-sprd: " Baolin Wang
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