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