Messages in this thread | | | Date | Mon, 6 Apr 2009 07:48:13 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [GIT PULL] Ext3 latency fixes |
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On Mon, 6 Apr 2009, Jens Axboe wrote: > > Well, either you are submitting a single piece of IO (in which case you > just want to unplug or directly submit as part of the submit_bio()), or > you are submitting several IOS (in which case you just want to unplug at > the end of the IO submission, before waiting).
That's not true.
The plugging is often across multiple threads. It didn't _use_ to be (we always unplugged at wait), but it is now. Nothing else explains why that patch by Ted makes such a big throughput thing, because the code did
ret = submit_bh(WRITE_SYNC, bh); wait_on_buffer(bh);
ie it very much submits a _single_ IO, and waits on it. If plugging made a difference, that means that unplugging was delayed so long that somebody else does IO too - ie it gets delayed past a wait event.
So according to your own rules, that submit_bh() _should_ use WRITE_SYNC, but something bad happens if it does. I'm not quite seeing _what_, though, unless there are multiple processes trying to dirty the _same_ buffer, and they win if they all can dirty it without doing IO on it in between (and then the write turns into just one write).
Linus
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