Re: Patch: Write Amplification Reduction Method (WARM)

Hi Pavan,

On 3/28/17 11:04 AM, Robert Haas wrote:
> On Mon, Mar 27, 2017 at 10:25 PM, Pavan Deolasee
> <pavan(dot)deolasee(at)gmail(dot)com> wrote:
>> On Tue, Mar 28, 2017 at 1:59 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> On Thu, Mar 23, 2017 at 2:47 PM, Pavan Deolasee
>>> <pavan(dot)deolasee(at)gmail(dot)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(at)pgmasters(dot)net