Messages in this thread | | | Date | Thu, 17 Apr 2008 10:57:00 +0200 | From | Jens Axboe <> | Subject | Re: [RESEND][RFC] BFQ I/O Scheduler |
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On Thu, Apr 17 2008, Pavel Machek wrote: > > On Thu, Apr 17 2008, Paolo Valente wrote: > > > Pavel Machek ha scritto: > > > > > > > >>In the first type of tests, to achieve a higher throughput than CFQ > > > >>(with the default 100 ms time slice), the maximum budget for BFQ > > > >>had to be set to at least 4k sectors. Using the same value for the > > > >> > > > > > > > >Hmm, 4k sectors is ~40 seconds worst case, no? That's quite long... > > > > > > > Actually, in the worst case among our tests, the aggregate throughput > > > with 4k sectors was ~ 20 MB/s, hence the time for 4k sectors ~ 4k * 512 > > > / 20M = 100 ms. > > > > That's not worse case, it is pretty close to BEST case. Worst case is 4k > > of sectors, with each being a 512b IO and causing a full stroke seek. > > For that type of workload, even a modern sata hard drive will be doing > > 500kb/sec or less. That's rougly a thousand sectors per seconds, so ~4 > > seconds worst case for 4k sectors. > > One seek is still 10msec on modern drive, right? So 4k seeks = > 40seconds, no? 4seconds would correspond to 1msec per seek, which > seems low.
I actually meant 4k ios, not 512b at that isn't really realistic. With 512b full device seeks, you are looking at probably 30kb/sec on a normal 7200rpm drive and that would be around a minute worst case time. The 4kb number of 500kb/sec may even be a bit too high, doing a quick test here shows a little less than 300kb/sec on this drive. So more than 4 seconds still, around 7-8s or so.
> writes with O_SYNC could force full seek on each request, right?
Writes generally work somewhat better due to caching, but doing O_DIRECT 512 byte reads all over the drive would exhibit worst case behaviour easily.
-- Jens Axboe
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