Re: Patch: Write Amplification Reduction Method (WARM) - Mailing list pgsql-hackers
| From | Pavan Deolasee |
|---|---|
| Subject | Re: Patch: Write Amplification Reduction Method (WARM) |
| Date | |
| Msg-id | CABOikdNxPDL=X2GcWqyAZdnVb=hb8vnKfvO8sBSfGd5MM6rydA@mail.gmail.com Whole thread Raw |
| In response to | Re: Patch: Write Amplification Reduction Method (WARM) (Robert Haas <robertmhaas@gmail.com>) |
| List | pgsql-hackers |
On Tue, Mar 28, 2017 at 8:34 PM, Robert Haas <robertmhaas@gmail.com> wrote:
On Mon, Mar 27, 2017 at 10:25 PM, Pavan Deolasee
<pavan.deolasee@gmail.com> wrote:
> On Tue, Mar 28, 2017 at 1:59 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Thu, Mar 23, 2017 at 2:47 PM, Pavan Deolasee
>> <pavan.deolasee@gmail.com> wrote:
>> > It's quite hard to say that until we see many more benchmarks. As author
>> > of
>> > the patch, I might have got repetitive with my benchmarks. But I've seen
>> > over 50% improvement in TPS even without chain conversion (6 indexes on
>> > a 12
>> > column table test).
>>
>> This seems quite mystifying. What can account for such a large
>> performance difference in such a pessimal scenario? It seems to me
>> that without chain conversion, WARM can only apply to each row once
>> and therefore no sustained performance improvement should be possible
>> -- unless rows are regularly being moved to new blocks, in which case
>> those updates would "reset" the ability to again perform an update.
>> However, one would hope that most updates get done within a single
>> block, so that the row-moves-to-new-block case wouldn't happen very
>> often.
>
> I think you're confusing between update chains that stay within a block vs
> HOT/WARM chains. Even when the entire update chain stays within a block, it
> can be made up of multiple HOT/WARM chains and each of these chains offer
> ability to do one WARM update. So even without chain conversion, every
> alternate update will be a WARM update. So the gains are perpetual.
You're right, I had overlooked that. But then I'm confused: how does
the chain conversion stuff help as much as it does? You said that you
got a 50% improvement from WARM, because we got to skip half the index
updates. But then you said with chain conversion you got an
improvement of more like 100%. However, I would think that on this
workload, chain conversion shouldn't save much. If you're sweeping
through the database constantly performing updates, the updates ought
to be a lot more frequent than the vacuums.
No?
Unfortunately these tests were done on different hardware, with different settings and even slightly different scale factors. So they may not be exactly comparable. But there is no doubt chain conversion will help to some extent. I'll repeat the benchmark with chain conversion turned off and report the exact difference.
Thanks,
Pavan
Pavan Deolasee http://www.2ndQuadrant.com/
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