Subject: Re: Patch: Write Amplification Reduction Method (WARM)
To: Pavan Deolasee <pavan.deolasee@gmail.com>
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Cc: Robert Haas <robertmhaas@gmail.com>, Bruce Momjian <bruce@momjian.us>,
 Amit Kapila <amit.kapila16@gmail.com>,
 Alvaro Herrera <alvherre@2ndquadrant.com>,
 Jaime Casanova <jaime.casanova@2ndquadrant.com>,
 Haribabu Kommi <kommi.haribabu@gmail.com>,
 Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
From: David Steele <david@pgmasters.net>
Message-ID: <9fcbd725-c522-c512-45ed-af6cd2ae4f71@pgmasters.net>
Date: Tue, 28 Mar 2017 11:32:59 -0400
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Hi Pavan,

On 3/28/17 11:04 AM, Robert Haas 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?

It appears that a patch is required to address Amit's review.  I have 
marked this as "Waiting for Author".

Thanks,
-- 
-David
david@pgmasters.net


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