> From: Fabio Checconi <fchecconi@xxxxxxxxx> > Date: Wed, Nov 19, 2008 11:17:01AM +0100 > > > From: Aaron Carroll <aaronc@xxxxxxxxxxxxxxxxxx> > > Date: Wed, Nov 19, 2008 12:52:42PM +1100 > > > > 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: > > > > If you look at bfq_update_hw_tag(), the logic introduced by the patch > you mention is still there; BFQ starts with ->hw_tag = 1, and updates it > every 32 valid samples. What changed WRT your patch, apart from the > number of samples, is that the condition for a sample to be valid is: > > bfqd->rq_in_driver + bfqd->queued >= 5 > > while in your patch it is: > > cfqd->rq_queued > 5 || cfqd->rq_in_driver > 5 > > We preferred the first one because that sum better reflects the number > of requests that could have been dispatched, and I don't think that this > is wrong. > > There is a problem, but it's not within the tag detection logic itself. > From some quick experiments, what happens is that when a process starts, > CFQ considers it seeky (*), BFQ doesn't. As a side effect BFQ does not > always dispatch enough requests to correctly detect tagging. > > At the first seek you cannot tell if the process is going to bee seeky > or not, and we have chosen to consider it sequential because it improved > fairness in some sequential workloads (the CIC_SEEKY heuristic is used > also to determine the idle_window length in [bc]fq_arm_slice_timer()). > > Anyway, we're dealing with heuristics, and they tend to favor some > workload over other ones. If recovering this thoughput loss is more > important than a transient unfairness due to short idling windows assigned > to sequential processes when they start, I've no problems in switching > the CIC_SEEKY logic to consider a process seeky when it starts. > > Thank you for testing and for pointing out this issue, we missed it > in our testing. > > > (*) to be correct, the initial classification depends on the position > of the first accessed sector. Sorry, I forgot the patch... This seems to solve the problem with your workload here, does it work for you? [ The magic number would not appear in a definitive fix... ] --- diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 83e90e9..e9b010f 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -1322,10 +1322,12 @@ static void bfq_update_io_seektime(struct bfq_data *bfqd, /* * Don't allow the seek distance to get too large from the - * odd fragment, pagein, etc. + * odd fragment, pagein, etc. The first request is not + * really a seek, but we consider a cic seeky on creation + * to make the hw_tag detection logic work better. */ - if (cic->seek_samples == 0) /* first request, not really a seek */ - sdist = 0; + if (cic->seek_samples == 0) + sdist = 8 * 1024 + 1; else if (cic->seek_samples <= 60) /* second&third seek */ sdist = min(sdist, (cic->seek_mean * 4) + 2*1024*1024); else _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
