Messages in this thread | | | Date | Mon, 8 Nov 2010 21:39:20 -0500 | From | Vivek Goyal <> | Subject | Re: [patch 2/3]cfq-iosched: schedule dispatch for noidle queue |
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On Tue, Nov 09, 2010 at 10:28:36AM +0800, Shaohua Li wrote:
[..] > > > > Why do we have to wait for all requests to finish in device? Will driver > > > > most likely not ask for next request when 1-2 requests have completed > > > > and at that time we should expire the queue if queue is no more marked > > > > as "noidle"? > > > The issue is a queue is idle just because it's the last queue of the > > > service tree. Then a new queue is added and the idled queue should not > > > idle now. we should preempt the idled queue soon. does this make sense > > > to you? > > > > If that's the case then you should just modify should_preempt() so that > > addition of a new queue could preempt an empty queue which has now become > > noidle. > > > > You have also modified cfq_completed_request() function, which will wake > > up the worker thread and then try to dispatch a request. IMHO, in practice > > driver asks for new request almost immediately and you don't gain much > > by this additional wakeup. > > > > So my point being, that we increased the code complexity for no visible > > performance improvement also increased thread wakeups resulting in more > > cpu consumption. > Ah, you are right, we only need modify should_preempt. Updated the patch as below. >
Thanks. Jens has already applied the patches on for-2.6.38/core branch of block tree. I think you shall have to generate an incremental patch which revert the bits introduced in cfq_completed_request().
Thanks Vivek
> > If there was a visible performance gain in your testing then it would have > > made sense but you said that you did not notice any improvements. Then > > why to increase the complexity. > I only test some workloads and can't do all tests, but this is an > obvious bug I thought. > > Thanks, > Shaohua > > Subject: cfq-iosched: preempt an idle queue if it should not be idle any more > > A queue is idle at cfq_dispatch_requests(), but it gets noidle later. Unless > other task explictly does unplug or all requests are drained, we will not > deliever requests to the disk even cfq_arm_slice_timer doesn't make the > queue idle. For example, cfq_should_idle() returns true because of > service_tree->count == 1, and then other queues are added. Note, I didn't > see obvious performance impacts so far with the patch, but just thought > this could be a problem. > > Signed-off-by: Shaohua Li <shaohua.li@intel.com> > > --- > block/cfq-iosched.c | 4 ++++ > 1 file changed, 4 insertions(+) > > Index: linux/block/cfq-iosched.c > =================================================================== > --- linux.orig/block/cfq-iosched.c 2010-11-09 10:20:38.000000000 +0800 > +++ linux/block/cfq-iosched.c 2010-11-09 10:20:54.000000000 +0800 > @@ -3265,6 +3265,10 @@ cfq_should_preempt(struct cfq_data *cfqd > if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq)) > return true; > > + /* An idle queue should not be idle now for some reason */ > + if (RB_EMPTY_ROOT(&cfqq->sort_list) && !cfq_should_idle(cfqd, cfqq)) > + return true; > + > if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq)) > return false; > > >
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