Messages in this thread |  | | | Date | Fri, 27 Jul 2007 23:51:43 +0200 (CEST) | | Subject | Re: swap-prefetch: A smart way to make good use of idle resources (was: updatedb) | | From | "Indan Zupancic" <> |
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On Fri, July 27, 2007 22:34, Arjan van de Ven wrote: > On Fri, July 27, 2007 21:43, Al Boldi wrote: >> IMHO, what everybody agrees on, is that swap-prefetch has a positive effect >> in some cases, and nobody can prove an adverse effect (excluding power >> consumption). The reason for this positive effect is also crystal clear: >> It prefetches from swap on idle into free memory, ie: it doesn't force > > the fact that there is free memory is ... strange. IN principle, Linux > keeps almost no memory free (except some emergency buffers) so that > things you swap in prematurely will BY DEFINITION go at the expense of > other things that could be there....
It's not strange, the use case here is if something memory hungry process is shut down it leaves behind a lot of free memory. Having things swapped out while there's free memory is strange, so swap prefetch fills it up again.
> also, they take up seek time (5 to 10 msec), so if you were to read > something else at the time you get additional latency.
If there's other disk activity swap prefetch shouldn't do much, so this isn't really true.
>> Conclusion: Either prove swap-prefetch is broken, or get this merged quick.
There are a whole lot of other requirements too than that it isn't broken (of which most are fulfilled, but anyway). One reason could be that there's a better solution out there for the problem swap prefetch tries to solve. That said, as swap prefetch is here now for a while and that other solution not it's not such a great argument.
Personally I think that a more generic solution would be better, one that prefetches the lastly evicted pages back in, not favouring either of swap or file data, like swap prefetch does now.
Greetings,
Indan
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