Re: [patch 0/4] [RFC] Another proportional weight IO controller

[Aaron Carroll <aaronc@gelato.unsw.edu.au>]

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@gelato.unsw.edu.au>
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
============================x8=============================