RE: Bluestore-Performance regression in the latest master

[Sage Weil <sweil@redhat.com>]

On Thu, 6 Oct 2016, Somnath Roy wrote:
> Mark,
> If you see the graph in the below link , you will see it is choppy and faster till 1 hour , beyond that it is degrading fast..
> If you see the left side graph that is holding the constant performance throughout for 10 hour (old master)
> Have you checked the latency with the shorter run with the latest master as well ? Even with the shorter run it is 4X more latency (even though aggregated throughput is bit more) for me..
> Anyways, I am trying to reproduce this in another setup just to eliminate HW issues in case..

You mentioned yesterday you had tried switching the rocksdb compaction 
mode.. could that be it?

sage

> 
> Thanks & Regards
> Somnath
> 
> -----Original Message-----
> From: Mark Nelson [mailto:mnelson@redhat.com] 
> Sent: Thursday, October 06, 2016 3:13 PM
> To: Somnath Roy; Sage Weil
> Cc: Ceph Development
> Subject: Re: Bluestore-Performance regression in the latest master
> 
> This is bluestore specific?  My guess is the cache sizes as well. 
> Bluestore on master is actually significantly faster for me in all of the tests I've thrown at it vs earlier this week, but my onode hit rate is still "decent".
> 
> I still see higher performance by increasing the min alloc size to 16k and then bumping up the onode cache size a bit with the memory savings.
> 
> Mark
> 
> On 10/06/2016 04:52 PM, Somnath Roy wrote:
> > I have increased the cache size at the point where it is started swapping after 30 min run or so , but, still I am seeing the huge latest difference.
> > Will try to find out what went wrong recently..
> >
> > Thanks & Regards
> > Somnath
> >
> > -----Original Message-----
> > From: Sage Weil [mailto:sweil@redhat.com]
> > Sent: Thursday, October 06, 2016 2:17 PM
> > To: Somnath Roy
> > Cc: Ceph Development
> > Subject: Re: Bluestore-Performance regression in the latest master
> >
> > On Thu, 6 Oct 2016, Somnath Roy wrote:
> >> Sage,
> >> I am seeing the performance with the latest master is not stable compare to 3 days old master. The peak performance is higher but it is jumping up and down. Analyzing further I suspect recent cache changes has an impact. While the performance is high , I am seeing lower disk reads (probably because of cache hits) , but the low point is much lower when it is missing the cache probably. You were saying on the standup about adding buffers in the cache in the write path , is that merged into the master ?
> >
> > The buffered writes by default is not merged:
> >
> > https://github.com/ceph/ceph/pull/11301
> >
> >> The following pull request for cache that got merged seems reasonable.
> >>
> >> https://github.com/ceph/ceph/pull/11295
> >
> > This cut the cache size down by 4x, plus whatever your sharding factor is/was.  Could that be it?
> >
> >> Any hunch what is causing this degradation recently ? Otherwise, I 
> >> need to dig down to find that out :-(
> >>
> >> See the graph in the following link for the performance difference.
> >>
> >> https://docs.google.com/spreadsheets/d/1JO3FaBbfT5jEdyYGB8EhHnAl5bAHe
> >> 0
> >> 6ojLE85mPzsK4/edit?usp=sharing
> >>
> >> The left side graph is with old master and right one is with latest master..Old one I ran for 10 hour and latest master I ran it for 2 hours.
> >> The new one has *4X more* latency as well..
> >
> > My guess is the cache size.  I can't think of what else would have much of an effect on latency...
> >
> > s
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