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SubjectRe: [Xen-devel] Backport request to stable of two performance related fixes for xen-blkfront (3.13 fixes to earlier trees)
Roger Pau Monné <> writes:

> On 10/06/14 15:19, Vitaly Kuznetsov wrote:
>> Vitaly Kuznetsov <> writes:
>>> Jiri Slaby <> 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:
>>> 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:
>> (same markers mean same bs, we get 860 MB/s here, patches make no
>> difference, result matches expectation)
>> 2) LVM Stripe:
>> (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:
>> (860 MB/sec again, patches worsen a bit overall throughput with 1-3
>> clients)
>> 2) LVM Stripe:
>> (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
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:

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.


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