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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