Fabio Checconi wrote:
> - To detect hw tagging in BFQ we consider a sample valid iff the
> number of requests that the scheduler could have dispatched (given
> by cfqd->rb_queued + cfqd->rq_in_driver, i.e., the ones still into
> the scheduler plus the ones into the driver) is higher than the
> CFQ_HW_QUEUE_MIN threshold. This obviously caused no problems
> during testing, but the way CFQ uses now seems a little bit
> strange.
BFQ's tag detection logic is broken in the same way that CFQ's used to
be. Explanation is in this patch:
============================x8============================
commit 45333d5a31296d0af886d94f1d08f128231cab8e
Author: Aaron Carroll <aaronc@xxxxxxxxxxxxxxxxxx>
Date: Tue Aug 26 15:52:36 2008 +0200
cfq-iosched: fix queue depth detection
CFQ's detection of queueing devices assumes a non-queuing device and detects
if the queue depth reaches a certain threshold. Under some workloads (e.g.
synchronous reads), CFQ effectively forces a unit queue depth, thus defeating
the detection logic. This leads to poor performance on queuing hardware,
since the idle window remains enabled.
This patch inverts the sense of the logic: assume a queuing-capable device,
and detect if the depth does not exceed the threshold.
============================x8=============================
BFQ seems better than CFQ at avoiding this problem though. Using the following fio
job, I can routinely trigger it for 10s or so before BFQ detects queuing.
============================x8=============================
[global]
direct=1
ioengine=sync
norandommap
randrepeat=0
filename=/dev/sdb
bs=16k
runtime=200
time_based
[reader]
rw=randread
numjobs=128
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