On Wed, 2006-03-22 at 07:48 +0000, Simon Riggs wrote:
> On Tue, 2006-03-21 at 17:47 -0500, Tom Lane wrote:
>
> > I'm fairly unconvinced about Simon's underlying premise --- that we
> > can't make good use of work_mem in sorting after the run building phase
> > --- anyway.
>
> We can make good use of memory, but there does come a point in final
> merging where too much is of no further benefit. That point seems to be
> at about 256 blocks per tape; patch enclosed for testing. (256 blocks
> per tape roughly doubles performance over 32 blocks at that stage).
>
> That is never the case during run building - more is always better.
>
> > If we cut back our memory usage
> Simon inserts the words: "too far"
> > then we'll be forcing a
> > significantly more-random access pattern to the temp file(s) during
> > merging, because we won't be able to pre-read as much at a time.
>
> Yes, thats right.
>
> If we have 512MB of memory that gives us enough for 2000 tapes, yet the
> initial runs might only build a few runs. There's just no way that all
> 512MB of memory is needed to optimise the performance of reading in a
> few tapes at time of final merge.
>
> I'm suggesting we always keep 2MB per active tape, or the full
> allocation, whichever is lower. In the above example that could release
> over 500MB of memory, which more importantly can be reused by subsequent
> sorts if/when they occur.
>
>
> Enclose two patches:
> 1. mergebuffers.patch allows measurement of the effects of different
> merge buffer sizes, current default=32
>
> 2. reassign2.patch which implements the two kinds of resource
> deallocation/reassignment proposed.
Missed couple of minor points in patch: reassign3.patch attached ro
completely replace reassign2.patch.
Recent test results show that with a 512MB test sort we can reclaim 97%
of memory during final merge with only a noise level (+2%) increase in
overall elapsed time. (Thats just an example, your mileage may vary). So
a large query would use and keep about 536MB memory rather than 1536MB.
Best Regards, Simon Riggs