Re: TODO : Allow parallel cores to be used by vacuumdb [ WIP ]

On 27/09/14 11:36, Gregory Smith wrote:
> On 9/26/14, 2:38 PM, Gavin Flower wrote:
>> Curious: would it be both feasible and useful to have multiple 
>> workers process a 'large' table, without complicating things too 
>> much?  The could each start at a different position in the file.
>
> Not really feasible without a major overhaul.  It might be mildly 
> useful in one rare case.  Occasionally I'll find very hot single 
> tables that vacuum is constantly processing, despite mostly living in 
> RAM because the server has a lot of memory.  You can set 
> vacuum_cost_page_hit=0 in order to get vacuum to chug through such a 
> table as fast as possible.
>
> However, the speed at which that happens will often then be limited by 
> how fast a single core can read from memory, for things in 
> shared_buffers.  That is limited by the speed of memory transfers from 
> a single NUMA memory bank.  Which bank you get will vary depending on 
> the core that owns that part of shared_buffers' memory, but it's only 
> one at a time.
>
> On large servers, that can be only a small fraction of the total 
> memory bandwidth the server is able to reach.  I've attached a graph 
> showing how this works on a system with many NUMA banks of RAM, and 
> this is only a medium sized system.  This server can hit 40GB/s of 
> memory transfers in total; no one process will ever see more than 8GB/s.
>
> If we had more vacuum processes running against the same table, there 
> would then be more situations where they were doing work against 
> different NUMA memory banks at the same time, therefore making faster 
> progress through the hits in shared_buffers possible. In the real 
> world, this situation is rare enough compared to disk-bound vacuum 
> work that I doubt it's worth getting excited over.  Systems with lots 
> of RAM where performance is regularly dominated by one big ugly table 
> are common though, so I wouldn't just rule the idea out as not useful 
> either.
>
Thanks for the very detailed reply of yours, and the comments from others.

Cheers,
Gavin