Messages in this thread | | | Subject | Re: [rfc][patch] remove racy sync_page? | From | Arjan van de Ven <> | Date | Wed, 31 May 2006 14:36:25 +0200 |
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On Wed, 2006-05-31 at 14:31 +0200, Helge Hafting wrote: > Linus Torvalds wrote: > > On Wed, 31 May 2006, Nick Piggin wrote: > > > >> The requests can only get merged if contiguous requests from the upper > >> layers come down, right? > >> > > > > It has nothing to do with merging. It has to do with IO patterns. > > > > Seeking. > > > > Seeking is damn expensive - much more so than command issue. People forget > > that sometimes. > > > > If you can sort the requests so that you don't have to seek back and > > forth, that's often a HUGE win. > > > > Yes, the requests will still be small, and yes, the IO might happen in 4kB > > chunks, but it happens a lot faster if you do it in a good elevator > > ordering and if you hit the track cache than if you seek back and forth. > > > This is correct, but doesn't really explain why plugging might be good. > > If requests go to disk immediately and the disk is able to keep > up with the seeks, then plugging doesn't help. This is a low-bandwith > case of course, but servicing each request immediately will > keep the latency lower.
only for writes. afaik for reads and other synchronous beasts we already unplug immediately anyway.
In addition unplugging happens after like 3 or 4 requests and after a few miliseconds, whatever comes first ... so you never wait really long (on a "what a seek costs" scale) anyway
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