Messages in this thread | | | Date | Sat, 12 Jul 2008 11:00:06 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH] x86_64: fix delayed signals |
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On Sat, 12 Jul 2008, Arjan van de Ven wrote: > > I see really bad delays on 32 bit as well, but they go away for me if I > do > echo 4096 > /sys/block/sda/queue/nr_requests
Hmm. I think the default is 128, and in many cases latencies should actually go up with bigger requests queues - especially if it means that you can have a lot more writes in front of the read. You see the opposite behaviour.
That could easily happen if the scheduler is crazy and lets writes use up all of the request queue, or if the limited queue means that it cannot effectively merge requests. But request merging should happen trivially for the contiguous 'dd' case almost regardless of queue size, so I wonder if something else is going on.
Ahh.. I see something _very_ suspicious.
Look at block/blk-core.c: get_request(). It starts throttling and batching requests when it gets
if (rl->count[rw]+1 >= queue_congestion_on_threshold(q)) {
and notice how this is independent of whether it's a read or a write (but it does count them separately). But on the wakeup path, it uses different limits for reads than for writes.
That batching looks pretty bogus for reads to begin with, and then behaving similarly on throttling but differently on wakup sounds bogus.
The blk_alloc_request() also ends up allocating all requests from one mempool, so if that mempool runs out (due to writes having used them all up), then those writes will block reads too, even though reads should have much higher priority.
I dunno. But there _has_ been a lot of churn in the different block queues over the last few months. I wouldn't be surprised at all if something got broken in the process. And as with filesystems, almost all performance tests are for throughput, not "bad latency" in the presense of other heavy IO.
Linus
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