RE: [Xen-devel] Backport request to stable of two performance related fixes for xen-blkfront (3.13 fixes to earlier trees)

From: Felipe Franciosi
Date: Fri Jun 20 2014 - 13:06:26 EST

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Hi all,

Vitaly and I just some hours having a look at his environment together and examining what was going on. Kudos for Vitaly for putting together the virtual meeting and preparing the whole environment for this!

The short version is that we don't believe the patches introduce a regression. There is one particular case that we measured where the throughput was a bit lower with the patches backported, but that scenario had several other factors affecting the workload as you can read below. For all other cases the throughput is either equivalent or higher when the patches are applied.

The patches should be taken for guests on the affected kernel versions.

The longer version:

We started by ensuring the system was stable with little variation between measurements by:
* Disabling hyper threading, turbo and C-States on the BIOS.
* Using Xen's performance governor (cores fixed to P0).
* Setting the maximum C-State to 1 (cores fixed to C0 and C1).
* Identity pinning dom0's vCPUs to NUMA node 0.
** vcpu0 -> pcpu0, vcpu1->pcpu1, vcpu2->pcpu2, vcpu3->pcpu3.
* Pinning guest's vCPUs to NUMA node 1.
** domU1vcpu0->pcpus4-7, domU2vcpu0->pcpus4-7, ...
* We used the NOOP scheduler within the guest (the Fusion-io and the device mapper block devices in dom0 don't register schedulers).

We also tried to identify all meaningful differences between Vitaly's setup and tests that I've been running. The main ones were:
* Vitaly is using HVM guests. (I've been testing on PV guests.)
* Vitaly's LVM configuration was different for his test disks:
** Vitaly's Fusion-io presented 2 SCSI devices which were presented to LVM as two physical volumes (PV).
** Both PVs were added to a single volume group (VG).
** Stripped logical volumes (using 4K stripes) were created on the VG to utilise both disks.
** I am using several SSDs on my test and treating them independently.

When we repeated his experiments on this configuration for a full sequential read workload with 1MB requests, we got:
Guests running 3.10 stock: Aggregated 1.7 GB/s
Guests running 3.10 +backports: Aggregated 1.6 GB/s

This is the only case where we identified the backports causing a regression (keep reading).

We made a few important observations:
* The dom0 CPU utilisation was high-ish for both scenarios:
** Cores 1-3 were at 70%, mostly in system time (blkback + fusion-io workers)
** Core 0 was at 100% and receiving all hardware interrupts
* The device mapper block devices receiving IO from blkback had an average request size of 4K.
** They were subsequently being merged back to the Fusion-io block devices.
** We identified this to be an effect of the 4K stripes at the logical volume level.

Next, we addressed the first difference between our environments and repeated the experiment with guests in PV mode, also trying 4M and 64K requests. On this experiment, there was no noticeable difference when using either kernel. As expected, with 64K requests the throughput was a bit lower, but still comparable between kernels. The throughput on the cases with 1M and 4M were identical.

Next, we addressed the second difference between our environments and recreated the LVM configuration as follows:
* One physical volume per Fusion-io device.
* One volume group per physical volume.
* Four logical volumes per volume group (without stripping, just using a linear table).
* Assigned one logical volume for each guest for the total of 8 guests.

When repeating the experiment one last time, we noticed the following:
* The CPU utilisation was lower for both HVM and PV guests.
** We believe this due the reduced stress caused by breaking and merging requests at the device mapper level.
* The requests reaching each logical volume were now 44K in size.
** We were not using indirect IO for this test.

On this modified LVM configuration, the kernel with the backports was actually faster than the stock 3.10. This was both for PV and HVM guests:
Guests running 3.10 stock: Aggregated 1.6 GB/s
Guests running 3.10 +backports: Aggregated 1.7 GB/s

In conclusion:

I believe the environment with only one SSD does not provide enough storage power to conclusively show the regression that Roger's patches are addressing. When looking at the measurements I did for the Ubuntu report, it is possible to observe the regression becomes noticeable for throughputs that are much higher than 1.7 GB/s:
https://launchpadlibrarian.net/176700111/saucy64.png
https://launchpadlibrarian.net/176700099/saucy64-backports.png

There is the weird case where the stripped LVM configuration forced some CPU contention in dom0 and, for HVM guests, the patches result in the throughput being slightly slower. We are open to ideas on this one.

All in all, the patches are important and can drastically improve the throughput of guests in the affected kernel range when the backend does not support persistent grants.

Thanks,
Felipe

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: 12 June 2014 16:33
> To: Felipe Franciosi
> Cc: Roger Pau Monne; xen-devel@xxxxxxxxxxxxxxxxxxxx; axboe@xxxxxxxxx;
> Greg KH; linux-kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx;
> jerry.snitselaar@xxxxxxxxxx; Jiri Slaby; Ronen Hod; Andrew Jones
> Subject: Re: [Xen-devel] Backport request to stable of two performance
> related fixes for xen-blkfront (3.13 fixes to earlier trees)
>
> Felipe Franciosi <felipe.franciosi@xxxxxxxxxx> writes:
>
> > Hi Vitaly,
> >
> > Are you able to test a 3.10 guest with and without the backport that
> > Roger sent? This patch is attached to an e-mail Roger sent on "22 May
> > 2014 13:54".
>
> Sure,
>
> Now I'm comparing d642daf637d02dacf216d7fd9da7532a4681cfd3 and
> 46c0326164c98e556c35c3eb240273595d43425d commits from
> git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
> (with and without two commits in question). The test is exactly the same as
> described before.
>
> The result is here:
> http://hadoop.ru/pubfiles/bug1096909/fusion/310_nopgrants_stripe.png
>
> as you can see 46c03261 (without patches) wins everywhere.
>
> >
> > Because your results are contradicting with what these patches are
> > meant to do, I would like to make sure that this isn't related to
> > something else that happened after 3.10.
>
> I still think Dom0 kernel and blktap/blktap3 is what make a difference
> between our test environments.
>
> >
> > You could also test Ubuntu Sancy guests with and without the patched
> > kernels provided by Joseph Salisbury on launchpad:
> > https://bugs.launchpad.net/bugs/1319003
> >
> > Thanks,
> > Felipe
> >
> >> -----Original Message-----
> >> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> >> Sent: 12 June 2014 13:01
> >> To: Roger Pau Monne
> >> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; axboe@xxxxxxxxx; Felipe
> >> Franciosi; Greg KH; linux-kernel@xxxxxxxxxxxxxxx;
> >> stable@xxxxxxxxxxxxxxx; jerry.snitselaar@xxxxxxxxxx; Jiri Slaby;
> >> Ronen Hod; Andrew Jones
> >> Subject: Re: [Xen-devel] Backport request to stable of two
> >> performance related fixes for xen-blkfront (3.13 fixes to earlier
> >> trees)
> >>
> >> Roger Pau MonnÃ <roger.pau@xxxxxxxxxx> writes:
> >>
> >> > On 10/06/14 15:19, Vitaly Kuznetsov wrote:
> >> >> Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes:
> >> >>
> >> >>> Jiri Slaby <jslaby@xxxxxxx> writes:
> >> >>>
> >> >>>> On 06/04/2014 07:48 AM, Greg KH wrote:
> >> >>>>> On Wed, May 14, 2014 at 03:11:22PM -0400, Konrad Rzeszutek Wilk
> >> wrote:
> >> >>>>>> Hey Greg
> >> >>>>>>
> >> >>>>>> This email is in regards to backporting two patches to stable
> >> >>>>>> that fall under the 'performance' rule:
> >> >>>>>>
> >> >>>>>> bfe11d6de1c416cea4f3f0f35f864162063ce3fa
> >> >>>>>> fbe363c476afe8ec992d3baf682670a4bd1b6ce6
> >> >>>>>
> >> >>>>> Now queued up, thanks.
> >> >>>>
> >> >>>> AFAIU, they introduce a performance regression.
> >> >>>>
> >> >>>> Vitaly?
> >> >>>
> >> >>> I'm aware of a performance regression in a 'very special' case
> >> >>> when ramdisks or files on tmpfs are being used as storage, I post
> >> >>> my results a while ago:
> >> >>> https://lkml.org/lkml/2014/5/22/164
> >> >>> I'm not sure if that 'special' case requires investigation and/or
> >> >>> should prevent us from doing stable backport but it would be nice
> >> >>> if someone tries to reproduce it at least.
> >> >>>
> >> >>> I'm going to make a bunch of tests with FusionIO drives and
> >> >>> sequential read to replicate same test Felipe did, I'll report as
> >> >>> soon as I have data (beginning of next week hopefuly).
> >> >>
> >> >> Turns out the regression I'm observing with these patches is not
> >> >> restricted to tmpfs/ramdisk usage.
> >> >>
> >> >> I was doing tests with Fusion-io ioDrive Duo 320GB (Dual Adapter)
> >> >> on HP ProLiant DL380 G6 (2xE5540, 8G RAM). Hyperthreading is
> >> >> disabled,
> >> >> Dom0 is pinned to CPU0 (cores 0,1,2,3) I run up to 8 guests with 1
> >> >> vCPU each, they are pinned to CPU1 (cores 4,5,6,7,4,5,6,7). I
> >> >> tried differed pinning (Dom0 to 0,1,4,5, DomUs to 2,3,6,7,2,3,6,7
> >> >> to balance NUMA, that doesn't make any difference to the results).
> >> >> I was testing on top of Xen-4.3.2.
> >> >>
> >> >> I was testing two storage configurations:
> >> >> 1) Plain 10G partitions from one Fusion drive (/dev/fioa) are
> >> >> attached to guests
> >> >> 2) LVM group is created on top of both drives (/dev/fioa,
> >> >> /dev/fiob), 10G logical volumes are created with striping
> >> >> (lvcreate -i2 ...)
> >> >>
> >> >> Test is done by simultaneous fio run in guests (rw=read, direct=1)
> >> >> for
> >> >> 10 second. Each test was performed 3 times and the average was
> taken.
> >> >> Kernels I compare are:
> >> >> 1) v3.15-rc5-157-g60b5f90 unmodified
> >> >> 2) v3.15-rc5-157-g60b5f90 with
> >> 427bfe07e6744c058ce6fc4aa187cda96b635539,
> >> >> bfe11d6de1c416cea4f3f0f35f864162063ce3fa, and
> >> >> fbe363c476afe8ec992d3baf682670a4bd1b6ce6 reverted.
> >> >>
> >> >> First test was done with Dom0 with persistent grant support
> >> >> (Fedora's
> >> >> 3.14.4-200.fc20.x86_64):
> >> >> 1) Partitions:
> >> >> http://hadoop.ru/pubfiles/bug1096909/fusion/315_pgrants_partitions
> >> >> .pn g (same markers mean same bs, we get 860 MB/s here, patches
> >> >> make no difference, result matches expectation)
> >> >>
> >> >> 2) LVM Stripe:
> >> >>
> http://hadoop.ru/pubfiles/bug1096909/fusion/315_pgrants_stripe.png
> >> >> (1715 MB/s, patches make no difference, result matches
> >> >> expectation)
> >> >>
> >> >> Second test was performed with Dom0 without persistent grants
> >> >> support (Fedora's 3.7.9-205.fc18.x86_64)
> >> >> 1) Partitions:
> >> >>
> http://hadoop.ru/pubfiles/bug1096909/fusion/315_nopgrants_partitions.
> >> >> png
> >> >> (860 MB/sec again, patches worsen a bit overall throughput with
> >> >> 1-3
> >> >> clients)
> >> >>
> >> >> 2) LVM Stripe:
> >> >>
> http://hadoop.ru/pubfiles/bug1096909/fusion/315_nopgrants_stripe.p
> >> >> ng (Here we see the same regression I observed with ramdisks and
> >> >> tmpfs files, unmodified kernel: 1550MB/s, with patches reverted:
> >> >> 1715MB/s).
> >> >>
> >> >> The only major difference with Felipe's test is that he was using
> >> >> blktap3 with XenServer and I'm using standard blktap2.
> >> >
> >> > Hello,
> >> >
> >> > I don't think you are using blktap2, I guess you are using blkback.
> >>
> >> Right, sorry for the confusion.
> >>
> >> > Also, running the test only for 10s and 3 repetitions seems too
> >> > low, I would probably try to run the tests for a longer time and do
> >> > more repetitions, and include the standard deviation also.
> >> >
> >> > Could you try to revert the patches independently to see if it's a
> >> > specific commit that introduces the regression?
> >>
> >> I did additional test runs. Now I'm comparing 3 kernels:
> >> 1) Unmodified v3.15-rc5-157-g60b5f90 - green color on chart
> >>
> >> 2) v3.15-rc5-157-g60b5f90 with
> >> bfe11d6de1c416cea4f3f0f35f864162063ce3fa
> >> and 427bfe07e6744c058ce6fc4aa187cda96b635539 reverted (so only
> >> fbe363c476afe8ec992d3baf682670a4bd1b6ce6 "xen-blkfront: revoke
> >> foreign access for grants not mapped by the backend" left) - blue
> >> color on chart
> >>
> >> 3) v3.15-rc5-157-g60b5f90 with all
> >> (bfe11d6de1c416cea4f3f0f35f864162063ce3fa,
> >> 427bfe07e6744c058ce6fc4aa187cda96b635539,
> >> fbe363c476afe8ec992d3baf682670a4bd1b6ce6) patches reverted - red
> >> color on chart.
> >>
> >> I test on top of striped LVM on 2 FusionIO drives, I do 3 repetitions
> >> for
> >> 30 seconds each.
> >>
> >> The result is here:
> >>
> http://hadoop.ru/pubfiles/bug1096909/fusion/315_nopgrants_20140612.pn
> >> g
> >>
> >> It is consistent with what I've measured with ramdrives and tmpfs files:
> >>
> >> 1) fbe363c476afe8ec992d3baf682670a4bd1b6ce6 "xen-blkfront: revoke
> >> foreign access for grants not mapped by the backend" brings us the
> >> regression. Bigger block size is - bigger the difference but the
> >> regression is observed with all block sizes > 8k.
> >>
> >> 2) bfe11d6de1c416cea4f3f0f35f864162063ce3fa "xen-blkfront: restore
> >> the non-persistent data path" brings us performance improvement but
> >> with conjunction with fbe363c476afe8ec992d3baf682670a4bd1b6ce6 it is
> >> still worse than the kernel without both patches.
> >>
> >> My Dom0 is Fedora's 3.7.9-205.fc18.x86_64. I can test on newer
> >> blkback, however I'm not aware of any way to disable persistent
> >> grants there (there is no regression when they're used).
> >>
> >> >
> >> > Thanks, Roger.
> >>
> >> Thanks,
> >>
> >> --
> >> Vitaly
>
> --
> Vitaly

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