Re: Move unused buffers to freelist

On Thursday, May 23, 2013 8:45 PM Robert Haas wrote:
> On Tue, May 21, 2013 at 3:06 AM, Amit Kapila <amit.kapila@huawei.com>
> wrote:
> >> Here are the results.  The first field in each line is the number of
> >> clients. The second number is the scale factor.  The numbers after
> >> "master" and "patched" are the median of three runs.
> 
> >>but overall, on both the read-only and
> >> read-write tests, I'm not seeing anything that resembles the big
> gains
> >> you reported.
> >
> > I have not generated numbers for read-write tests, I will check that
> once.
> > For read-only tests, the performance increase is minor and different
> from
> > what I saw.
> > Few points which I could think of for difference in data:
> >
> > 1. In my test's I always observed best data when number of
> clients/threads
> > are equal to number of cores which in your case should be at 16.
> 
> Sure, but you also showed substantial performance increases across a
> variety of connection counts, whereas I'm seeing basically no change
> at any connection count.
> > 2. I think for scale factor 100 and 300, there should not be much
> > performance increase, as for them they should mostly get buffer from
> > freelist inspite of even bgwriter adds to freelist or not.
> 
> I agree.
> 
> > 3. In my tests variance is for shared buffers, database size is
> always less
> > than RAM (Scale Factor -1200, approx db size 16~17GB, RAM -24 GB),
> but due
> > to variance in shared buffers, it can lead to I/O.
> 
> Not sure I understand this.

What I wanted to say is that all your tests was on same shared buffer
configuration 8GB, where as in my tests I was trying to vary shared buffers
as well.
However this is not important point, as it should show performance gain on
configuration you ran, if there is any real benefit of this patch.
> > 4. Each run is of 20 minutes, not sure if this has any difference.
> 
> I've found that 5-minute tests are normally adequate to identify
> performance changes on the pgbench SELECT-only workload.
> 
> >> Tests were run on a 16-core, 64-hwthread PPC64 machine provided to
> the
> >> PostgreSQL community courtesy of IBM.  Fedora 16, Linux kernel
> 3.2.6.
> >
> > To think about the difference in your and my runs, could you please
> tell me
> > about below points
> > 1. What is RAM in machine.
> 
> 64GB
> 
> > 2. Are number of threads equal to number of clients.
> 
> Yes.
> 
> > 3. Before starting tests I have always done pre-warming of buffers
> (used
> > pg_prewarm written by you last year), is it same for above read-only
> tests.
> 
> No, I did not use pg_prewarm.  But I don't think that should matter
> very much.  First, the data was all in the OS cache.  Second, on the
> small scale factors, everything should end up in cache pretty quickly
> anyway.  And on the large scale factors, well, you're going to be
> churning shared_buffers anyway, so pg_prewarm is only going to affect
> the very beginning of the test.
> 
> > 4. Can you please once again run only the test where you saw
> variation(8
> > clients @ scale> factor 1000 on master), because I have also seen
> that
> > performance difference is very good for certain
> >    configurations(Scale Factor, RAM, Shared Buffers)
> 
> I can do this if I get a chance, but I don't really see where that's
> going to get us.  It seems pretty clear to me that there's no benefit
> on these tests from this patch.  So either one of us is doing the
> benchmarking incorrectly, or there's some difference in our test
> environments that is significant, but none of the proposals you've
> made so far seem to me to explain the difference.

Sorry for requesting you to run again without any concrete point.
I realized after reading data you posted more carefully that the reading was
just some m/c problem or something else, but actually there is no gain.
After your post, I had tried with various configurations on different m/c,
but till now I am not able see the performance gain as was shown in my
initial mail.
Infact I had tried on same m/c as well, it some times give good data. I will
update you if I get any concrete reason and results.

> > Apart from above, I had one more observation during my investigation
> to find
> > why in some cases, there is a small dip:
> > 1. Many times, it finds the buffer in free list is not usable, means
> it's
> > refcount or usage count is not zero, due to which it had to spend
> more time
> > under BufFreelistLock.
> >    I had not any further experiments related to this finding like if
> it
> > really adds any overhead.
> >
> > Currently I am trying to find reasons for small dip of performance
> and see
> > if I could do something to avoid it. Also I will run tests with
> various
> > configurations.
> >
> > Any other suggestions?
> 
> Well, I think that the code in SyncOneBuffer is not really optimal.
> In some cases you actually lock and unlock the buffer header an extra
> time, which seems like a whole lotta extra overhead.  In fact, I don't
> think you should be modifying SyncOneBuffer() at all, because that
> affects not only the background writer but also checkpoints.
> Presumably it is not right to put every unused buffer on the free list
> when we checkpoint.
> 
> Instead, I suggest modifying BgBufferSync, specifically this part right
> here:
> 
>         else if (buffer_state & BUF_REUSABLE)
>             reusable_buffers++;
> 
> What I would suggest is that if the BUF_REUSABLE flag is set here, use
> that as the trigger to do StrategyMoveBufferToFreeListEnd().  

I think at this point also we need to lock buffer header to check refcount
and usage_count before moving to freelist, or do you think it is not
required?

> That's
> much simpler than the logic that you have now, and I think it's also
> more efficient and more correct.

Sure, I will try the logic suggested by you.

With Regards,
Amit Kapila.