Re: MVCC overheads

Please see comment at the bottom of this post.

On 08/07/16 10:48, Pete Stevenson wrote:
> Good info, thanks for the note. Agreed that it is difficult to pull
> things apart to isolate these features for offload — so actually
> running experiments with offload is not possible, as you point out
> (and for other reasons).
>
> Maybe I could figure out the lines of code that add versions into a
> table and then those that collect old versions (they do get collected,
> right?). Anyway, thought being I could profile while running TPC-C or
> similar. I was hoping that someone might be able to jump on this with
> a response that they already did something similar. I know that
> Stonebraker has done some analysis along these lines, but I’m looking
> for an independent result that confirms (or not) his work.
>
> Thank you,
> Pete Stevenson
>
>
>> On Jul 7, 2016, at 3:43 PM, Simon Riggs <simon(at)2ndquadrant(dot)com
>> <mailto:simon(at)2ndquadrant(dot)com>> wrote:
>>
>> On 7 July 2016 at 20:50, Pete Stevenson <etep(dot)nosnevets(at)gmail(dot)com
>> <mailto:etep(dot)nosnevets(at)gmail(dot)com>> wrote:
>>
>> Hi Simon -
>>
>> Thanks for the note. I think it's fair to say that I didn't
>> provide enough context, so let me try and elaborate on my question.
>>
>> I agree, MVCC is a benefit. The research angle here is about
>> enabling MVCC with hardware offload. Since I didn't explicitly
>> mention it, the offload I refer to will respect all consistency
>> guarantees also.
>>
>> It is the case that for the database to implement MVCC it must
>> provide consistent read to multiple different versions of data,
>> i.e. depending on the version used at transaction start. I'm not
>> an expert on postgresql internals, but this must have some cost.
>> I think the cost related to MVCC guarantees can roughly be
>> categorized as: creating new versions (linking them in), version
>> checking on read, garbage collecting old versions, and then there
>> is an additional cost that I am interested in (again not claiming
>> it is unnecessary in any sense) but there is a cost to generating
>> the log.
>>
>> Thanks, by the way, for the warning about lab vs. reality. That's
>> why I'm asking this question here. I want to keep the
>> hypothetical tagged as such, but find defensible and realistic
>> metrics where those exist, i.e. in this instance, we do have a
>> database that can use MVCC. It should be possible to figure out
>> how much work goes into maintaining that property.
>>
>>
>> PostgreSQL uses a no overwrite storage mechanism, so any additional
>> row versions are maintained in the same table alongside other rows.
>> The MVCC actions are mostly mixed in with other aspects of the
>> storage, so not isolated much for offload.
>>
>> Oracle has a different mechanism that does isolate changed row
>> versions into a separate data structure, so would be much more
>> amenable to offload than PostgreSQL, in its current form.
>>
>> Maybe look at SLRUs (clog etc) as a place to offload something?
>>
>> --
>> Simon Riggs http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/>
>> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>
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Cheers,
Gavin