From: Ignat Korchagin <ignat@cloudflare.com>
To: Mike Snitzer <snitzer@redhat.com>,
Damien Le Moal <Damien.LeMoal@wdc.com>
Cc: "kernel-team@cloudflare.com" <kernel-team@cloudflare.com>,
"dm-crypt@saout.de" <dm-crypt@saout.de>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"dm-devel@redhat.com" <dm-devel@redhat.com>,
Mikulas Patocka <mpatocka@redhat.com>,
"agk@redhat.com" <agk@redhat.com>
Subject: Re: [RFC PATCH 0/1] dm-crypt excessive overhead
Date: Wed, 24 Jun 2020 09:02:39 +0100 [thread overview]
Message-ID: <CALrw=nE4Wf3mBqb9BvaEu32aSpAuuGsbKmPBReOHRze-0jyfkw@mail.gmail.com> (raw)
In-Reply-To: <20200624052209.GB23205@redhat.com>
On Wed, Jun 24, 2020 at 6:22 AM Mike Snitzer <snitzer@redhat.com> wrote:
>
> On Wed, Jun 24 2020 at 12:54am -0400,
> Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
>
> > On 2020/06/24 0:23, Mike Snitzer wrote:
> > > On Tue, Jun 23 2020 at 11:07am -0400,
> > > Ignat Korchagin <ignat@cloudflare.com> wrote:
> > >
> > >> Do you think it may be better to break it in two flags: one for read
> > >> path and one for write? So, depending on the needs and workflow these
> > >> could be enabled independently?
> > >
> > > If there is a need to split, then sure. But I think Damien had a hard
> > > requirement that writes had to be inlined but that reads didn't _need_
> > > to be for his dm-zoned usecase. Damien may not yet have assessed the
> > > performance implications, of not have reads inlined, as much as you
> > > have.
> >
> > We did do performance testing :)
> > The results are mixed and performance differences between inline vs workqueues
> > depend on the workload (IO size, IO queue depth and number of drives being used
> > mostly). In many cases, inlining everything does really improve performance as
> > Ignat reported.
> >
> > In our testing, we used hard drives and so focused mostly on throughput rather
> > than command latency. The added workqueue context switch overhead and crypto
> > work latency compared to typical HDD IO times is small, and significant only if
> > the backend storage as short IO times.
> >
> > In the case of HDDs, especially for large IO sizes, inlining crypto work does
> > not shine as it prevents an efficient use of CPU resources. This is especially
> > true with reads on a large system with many drives connected to a single HBA:
> > the softirq context decryption work does not lend itself well to using other
> > CPUs that did not receive the HBA IRQ signaling command completions. The test
> > results clearly show much higher throughputs using dm-crypt as is.
> >
> > On the other hand, inlining crypto work significantly improves workloads of
> > small random IOs, even for a large number of disks: removing the overhead of
> > context switches allows faster completions, allowing sending more requests to
> > the drives more quickly, keeping them busy.
> >
> > For SMR, the inlining of write requests is *mandatory* to preserve the issuer
> > write sequence, but encryption work being done in the issuer context (writes to
> > SMR drives can only be O_DIRECT writes), efficient CPU resource usage can be
> > achieved by simply using multiple writer thread/processes, working on different
> > zones of different disks. This is a very reasonable model for SMR as writes into
> > a single zone have to be done under mutual exclusion to ensure sequentiality.
> >
> > For reads, SMR drives are essentially exactly the same as regular disks, so
> > as-is or inline are both OK. Based on our performance results, allowing the user
> > to have the choice of inlining or not reads based on the target workload would
> > be great.
> >
> > Of note is that zone append writes (emulated in SCSI, native with NVMe) are not
> > subject to the sequential write constraint, so they can also be executed either
> > inline or asynchronously.
> >
> > > So let's see how Damien's work goes and if he trully doesn't need/want
> > > reads to be inlined then 2 flags can be created.
> >
> > For SMR, I do not need inline reads, but I do want the user to have the
> > possibility of using this setup as that can provide better performance for some
> > workloads. I think that splitting the inline flag in 2 is exactly what we want:
> >
> > 1) For SMR, the write-inline flag can be automatically turned on when the target
> > device is created if the backend device used is a host-managed zoned drive (scsi
> > or NVMe ZNS). For reads, it would be the user choice, based on the target workload.
> > 2) For regular block devices, write-inline only, read-inline only or both would
> > be the user choice, to optimize for their target workload.
> >
> > With the split into 2 flags, my SMR support patch becomes very simple.
>
> OK, thanks for all the context. Was a fun read ;)
>
> SO let's run with splitting into 2 flags. Ignat would you be up to
> tweaking your patch to provide that and post a v2?
>
> An added bonus would be to consolidate your 0/1 and 1/1 patch headers,
> and add in the additional answers you provided in this thread to help
> others understand the patch (mainly some more detail about why tasklet
> is used).
Yes, will do
> Thanks,
> Mike
>
next prev parent reply other threads:[~2020-06-24 8:02 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-19 16:41 [RFC PATCH 0/1] dm-crypt excessive overhead Ignat Korchagin
2020-06-19 16:41 ` [RFC PATCH 1/1] Add DM_CRYPT_FORCE_INLINE flag to dm-crypt target Ignat Korchagin
2020-06-24 5:04 ` [dm-crypt] " Eric Biggers
2020-06-24 5:21 ` [dm-devel] " Damien Le Moal
2020-06-24 5:27 ` Eric Biggers
2020-06-24 6:46 ` Damien Le Moal
2020-06-24 7:24 ` Damien Le Moal
2020-06-24 7:49 ` Damien Le Moal
2020-06-24 8:24 ` Ignat Korchagin
2020-06-24 16:24 ` Eric Biggers
2020-06-24 17:00 ` Ignat Korchagin
2020-06-24 5:12 ` [dm-devel] " Damien Le Moal
2020-06-19 16:55 ` [RFC PATCH 0/1] dm-crypt excessive overhead Mike Snitzer
2020-06-19 18:39 ` Mikulas Patocka
2020-06-19 19:44 ` Ignat Korchagin
2020-06-20 1:23 ` Herbert Xu
2020-06-20 19:36 ` Mikulas Patocka
2020-06-20 21:02 ` Ignat Korchagin
2020-06-23 15:33 ` Mike Snitzer
2020-06-23 16:24 ` Ignat Korchagin
2020-06-24 0:23 ` Herbert Xu
2020-06-22 0:45 ` [dm-devel] " Damien Le Moal
2020-06-22 7:55 ` Ignat Korchagin
2020-06-22 8:08 ` Damien Le Moal
2020-06-23 15:01 ` Mike Snitzer
2020-06-23 15:07 ` Ignat Korchagin
2020-06-23 15:22 ` Mike Snitzer
2020-06-24 4:54 ` [dm-devel] " Damien Le Moal
2020-06-24 5:22 ` Mike Snitzer
2020-06-24 8:02 ` Ignat Korchagin [this message]
2020-06-24 4:28 ` Damien Le Moal
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