Re: regression in page writeback

[Wu Fengguang]

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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