Messages in this thread | | | Date | Thu, 1 Oct 2009 17:54:38 -0400 | From | Theodore Tso <> | Subject | Re: regression in page writeback |
| |
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().
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. :-)
- Ted
| |