On Tue, Apr 19, 2016 at 8:41 PM, Kevin Grittner <kgri...@gmail.com> wrote:
> On Tue, Apr 19, 2016 at 9:57 AM, Amit Kapila <amit.kapil...@gmail.com>
> > On Sun, Apr 17, 2016 at 2:26 AM, Andres Freund <and...@anarazel.de>
> >>
> >> On 2016-04-16 16:44:52 -0400, Noah Misch wrote:
> >> > That is more controversial than the potential ~2% regression for
> >> > old_snapshot_threshold=-1.  Alvaro[2] and Robert[3] are okay
> >> > that way, and Andres[4] is not.
> >>
> >> FWIW, I could be kinda convinced that it's temporarily ok, if there'd
> >> a clear proposal on the table how to solve the scalability issue around
> >> MaintainOldSnapshotTimeMapping().
> >
> > It seems that for read-only workloads, MaintainOldSnapshotTimeMapping()
> > takes EXCLUSIVE LWLock which seems to be a probable reason for a
> > regression.  Now, here the question is do we need to acquire that lock
> > xmin is not changed since the last time value of
> > oldSnapshotControl->latest_xmin is updated or xmin is lesser than equal
> > oldSnapshotControl->latest_xmin?
> > If we don't need it for above cases, I think it can address the
> > regression to a good degree for read-only workloads when the feature is
> > enabled.
> Thanks, Amit -- I think something along those lines is the right
> solution to the scaling issues when the feature is enabled.

I have tried attached patch along the above lines and it seems that it
addresses performance regression to a good degree when feature is enabled
at moderate client-count like 32, but still more needs to be done for
somewhat higher client-count like 64.

Performance data is for median of 3, 5 min runs of read-only workload -
pgbench -c $client_count -j $client_count -T 300 -M prepared -S postgres

o_s_t - old_snapshot_threshold

Client_Count/Patch_Ver 32 64
HEAD (o_s_t = -1) 354077 552063
HEAD (o_s_t = 1) 92809 55847
Patch (o_s_t = 1) 319759 191741

If you think that attached patch is correct functionality wise, then I
think we can go-ahead with it and then investigate what more can be
improved.  I think newly introduced spinlocks might be the reason of
performance degradation at higher client-count, if that turns out to be
true, then I think we can replace them with atomics, once Andres's patch
for completing the 64-bit atomics implementation is committed.

m/c details used for performance testing
Architecture:          ppc64le
Byte Order:            Little Endian
CPU(s):                192
On-line CPU(s) list:   0-191
Thread(s) per core:    8
Core(s) per socket:    1
Socket(s):             24
NUMA node(s):          4
Model:                 IBM,8286-42A
L1d cache:             64K
L1i cache:             32K
L2 cache:              512K
L3 cache:              8192K
NUMA node0 CPU(s):     0-47
NUMA node1 CPU(s):     48-95
NUMA node2 CPU(s):     96-143
NUMA node3 CPU(s):     144-191

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

Attachment: old_snapshot_threshold_perf_issue_v1.patch
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