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

["Roger Pau Monné" <roger.pau@citrix.com>]

On 10/06/14 15:19, Vitaly Kuznetsov wrote:
> Vitaly Kuznetsov <vkuznets@redhat.com> writes:
> 
>> Jiri Slaby <jslaby@suse.cz> 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.png
> (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.png
> (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.
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?

Thanks, Roger.