Re: swap-prefetch: A smart way to make good use of idle resources (was: updatedb)

["Indan Zupancic" <indan@nul.nu>]

On Sat, July 28, 2007 00:06, Arjan van de Ven wrote:
> On Fri, 2007-07-27 at 23:51 +0200, Indan Zupanci
>> > 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.
>
> how do you know there will be other activity? You start the IO and that
> basically blacks out the disk for 5 to 10 ms. If the "real" IO gets
> submitted in that time you add latency. You cannot predict that IO
> happening or not happening.

Ah, in that way. Yes, you right about that (though NCQ might help then?),
but that's true for all disk activity. Though I think swap prefetch didn't want
to run when there was CPU activity, so that would reduce the chance that
new IO is submitted right at that moment. I think in practice this isn't worth
worrying about, the real issue is the extra disk activity in the first place.

Greetings,

Indan