| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Mark Mielke <mark(at)mark(dot)mielke(dot)cc> |
| Cc: | Jeff Davis <pgsql(at)j-davis(dot)com>, Michał Zaborowski <michal(dot)zaborowski(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Sorting Improvements for 8.4 |
| Date: | 2007-12-20 01:02:30 |
| Message-ID: | 1549.1198112550@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Mark Mielke <mark(at)mark(dot)mielke(dot)cc> writes:
> Jeff Davis wrote:
>> Also, there is probably a lot of memory copying going on, and that
>> probably destroys a lot of the effectiveness of L2 caching. When L2
>> caching is ineffective, the CPU spends a lot of time just waiting on
>> memory. In that case, it's better to have P threads of execution all
>> waiting on memory operations in parallel.
>>
> I didn't consider the high throughput / high latency effect. This could
> be true if the CPU prefetch isn't effective enough.
Note that if this is the argument, then there's a ceiling on the speedup
you can expect to get: it's just the extent of mismatch between the CPU
and memory speeds. I can believe that suitable test cases would show
2X improvement for 2 threads, but it doesn't follow that you will get
10X improvement with 10 threads, or even 4X with 4.
regards, tom lane
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