[Gluster-devel] performance issues Manoj found in EC testing

Pranith Kumar Karampuri pkarampu at redhat.com
Tue Jun 28 06:08:21 UTC 2016

On Tue, Jun 28, 2016 at 10:21 AM, Poornima Gurusiddaiah <pgurusid at redhat.com
> wrote:

> Regards,
> Poornima
> ------------------------------
> *From: *"Pranith Kumar Karampuri" <pkarampu at redhat.com>
> *To: *"Xavier Hernandez" <xhernandez at datalab.es>
> *Cc: *"Gluster Devel" <gluster-devel at gluster.org>
> *Sent: *Monday, June 27, 2016 5:48:24 PM
> *Subject: *Re: [Gluster-devel] performance issues Manoj found in EC
> testing
> On Mon, Jun 27, 2016 at 12:42 PM, Pranith Kumar Karampuri <
> pkarampu at redhat.com> wrote:
>> On Mon, Jun 27, 2016 at 11:52 AM, Xavier Hernandez <xhernandez at datalab.es
>> > wrote:
>>> Hi Manoj,
>>> I always enable client-io-threads option for disperse volumes. It
>>> improves performance sensibly, most probably because of the problem you
>>> have detected.
>>> I don't see any other way to solve that problem.
>> I agree. Updated the bug with same info.
>>> I think it would be a lot better to have a true thread pool (and maybe
>>> an I/O thread pool shared by fuse, client and server xlators) in
>>> libglusterfs instead of the io-threads xlator. This would allow each xlator
>>> to decide when and what should be parallelized in a more intelligent way,
>>> since basing the decision solely on the fop type seems too simplistic to me.
>>> In the specific case of EC, there are a lot of operations to perform for
>>> a single high level fop, and not all of them require the same priority.
>>> Also some of them could be executed in parallel instead of sequentially.
>> I think it is high time we actually schedule(for which release) to get
>> this in gluster. May be you should send out a doc where we can work out
>> details? I will be happy to explore options to integrate io-threads,
>> syncop/barrier with this infra based on the design may be.
> I was just thinking why we can't reuse synctask framework. It already
> scales up/down based on the tasks. At max it uses 16 threads. Whatever we
> want to be executed in parallel we can create a synctask around it and run
> it. Would that be good enough?
> Yes, synctask framework can be preferred over io-threads, else it would
> mean 16 synctask threads + 16(?) io-threads for one instance of mount, this
> will blow out the gfapi clients if they have many mounts from the same
> process. Also using synctask would mean code changes in EC?

Yes it will need some changes but I don't think they are big changes. I
think the functions to decode/encode already exist. We just to need to move
encoding/decoding as tasks and run as synctasks.

      Long time back we chatted a bit about synctask code and you wanted
the scheduling to happen by kernel or something. Apart from that do you see
any other issues? At least if the tasks are synchronous i.e. nothing goes
out the wire, task scheduling = thread scheduling by kernel and it works
exactly like thread-pool you were referring to. It does multi-tasking only
if the tasks are asynchronous in nature.

>>> Xavi
>>> On 25/06/16 19:42, Manoj Pillai wrote:
>>>> ----- Original Message -----
>>>>> From: "Pranith Kumar Karampuri" <pkarampu at redhat.com>
>>>>> To: "Xavier Hernandez" <xhernandez at datalab.es>
>>>>> Cc: "Manoj Pillai" <mpillai at redhat.com>, "Gluster Devel" <
>>>>> gluster-devel at gluster.org>
>>>>> Sent: Thursday, June 23, 2016 8:50:44 PM
>>>>> Subject: performance issues Manoj found in EC testing
>>>>> hi Xavi,
>>>>>           Meet Manoj from performance team Redhat. He has been testing
>>>>> EC
>>>>> performance in his stretch clusters. He found some interesting things
>>>>> we
>>>>> would like to share with you.
>>>>> 1) When we perform multiple streams of big file writes(12 parallel dds
>>>>> I
>>>>> think) he found one thread to be always hot (99%CPU always). He was
>>>>> asking
>>>>> me if fuse_reader thread does any extra processing in EC compared to
>>>>> replicate. Initially I thought it would just lock and epoll threads
>>>>> will
>>>>> perform the encoding but later realized that once we have the lock and
>>>>> version details, next writes on the file would be encoded in the same
>>>>> thread that comes to EC. write-behind could play a role and make the
>>>>> writes
>>>>> come to EC in an epoll thread but we saw consistently there was just
>>>>> one
>>>>> thread that is hot. Not multiple threads. We will be able to confirm
>>>>> this
>>>>> in tomorrow's testing.
>>>>> 2) This is one more thing Raghavendra G found, that our current
>>>>> implementation of epoll doesn't let other epoll threads pick messages
>>>>> from
>>>>> a socket while one thread is processing one message from that socket.
>>>>> In
>>>>> EC's case that can be encoding of the write/decoding read. This will
>>>>> not
>>>>> let replies of operations on different files to be processed in
>>>>> parallel.
>>>>> He thinks this can be fixed for 3.9.
>>>>> Manoj will be raising a bug to gather all his findings. I just wanted
>>>>> to
>>>>> introduce him and let you know the interesting things he is finding
>>>>> before
>>>>> you see the bug :-).
>>>>> --
>>>>> Pranith
>>>> Thanks, Pranith :).
>>>> Here's the bug: https://bugzilla.redhat.com/show_bug.cgi?id=1349953
>>>> Comparing EC and replica-2 runs, the hot thread is seen in both cases,
>>>> so
>>>> I have not opened this as an EC bug. But initial impression is that
>>>> performance impact for EC is particularly bad (details in the bug).
>>>> -- Manoj
>> --
>> Pranith
> --
> Pranith
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