Messages in this thread | | | Date | Fri, 2 Oct 2009 10:55:02 +0800 | From | Wu Fengguang <> | Subject | Re: regression in page writeback |
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On Fri, Oct 02, 2009 at 05:54:38AM +0800, Theodore Ts'o wrote: > On Thu, Oct 01, 2009 at 11:14:29PM +0800, Wu Fengguang wrote: > > Yes and no. Yes if the queue was empty for the slow device. No if the > > queue was full, in which case IO submission speed = IO complete speed > > for previously queued requests. > > > > So wbc.timeout will be accurate for IO submission time, and mostly > > accurate for IO completion time. The transient queue fill up phase > > shall not be a big problem? > > So the problem is if we have a mixed workload where there are lots > large contiguous writes, and lots of small writes which are fsync'ed() > --- for example, consider the workload of copying lots of big DVD > images combined with the infamous firefox-we-must-write-out-300-megs-of- > small-random-writes-and-then-fsync-them-on-every-single-url-click-so- > that-every-last-visited-page-is-preserved-for-history-bar-autocompletion > workload. The big writes, if the are contiguous, could take 1-2 seconds > on a very slow, ancient laptop disk, and that will hold up any kind of > small synchornous activities --- such as either a disk read or a firefox- > triggered fsync().
Yes, that's a problem. The SYNC/ASYNC elevator queues can help here.
In IO submission paths, fsync writes will not be blocked by non-sync writes because __filemap_fdatawrite_range() starts foreground sync for the inode. Without the congestion backoff, it will now have to compete queue with bdi-flush. Should not be a big problem though.
There's still the problem of IO submission time != IO completion time, due to fluctuations of randomness and more. However that's a general and unavoidable problem. Both the wbc.timeout scheme and the "wbc.nr_to_write based on estimated throughput" scheme are based on _past_ requests and it's simply impossible to have a 100% accurate scheme. In principle, wbc.timeout will only be inferior at IO startup time. In the steady state of 100% full queue, it is actually estimating the IO throughput implicitly :)
> That's why the IO completion time matters; it causes latency problems > for slow disks and mixed large and small write workloads. It was the > original reason for the 1024 MAX_WRITEBACK_PAGES, which might have > made sense 10 years ago back when disks were a lot slower. One of the > advantages of an auto-tuning algorithm, beyond auto-adjusting for > different types of hardware, is that we don't need to worry about > arbitrary and magic caps beocoming obsolete due to technological > changes. :-)
Yeah, I'm a big fan of auto-tuning :)
Thanks, Fengguang
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