Re: regression in page writeback

[Wu Fengguang <fengguang.wu@intel.com>]

On Sat, Oct 03, 2009 at 01:26:20AM +0800, Theodore Ts'o wrote:
> On Fri, Oct 02, 2009 at 04:19:53PM +0800, Wu Fengguang wrote:
> > > > The big writes, if they 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.
> 
> The SYNC/ASYNC queues will partially help, up to the whatever the
> largest I/O that can issued as a single chunk times the queue depth
> for those disks that support NCQ. 
> 
> > > 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 :)
> > 
> > Another difference between wbc.timeout and adaptive wbc.nr_to_write
> > is, when there comes many _read_ requests or fsync, these SYNC rw
> > requests will significant lower the ASYNC writeback throughput, if
> > it's not completely stalled. So with timeout, the inode will be
> > aborted with few pages written; with nr_to_write, the inode will be
> > written a good number of pages, at the cost of taking up long time.
> > 
> > IMHO the nr_to_write behavior seems more efficient. What do you think?
> 
> I agree, adaptively changing nr_to_write seems like the right thing to

I'd like to estimate the writeback throughput in bdi_writeback_wakeup(),
where the queue is not starved and the estimation would reflect the max
device capability (unless there are busy reads, in which case we need
lower nr_to_write anyway).

> do.  For bonus points, we could also monitor how often synchronous I/O
> operations are happening, allow nr_to_write to go up by some amount if
> there aren't many synchronous operations happening at the moment.  So
> that might be another opportunity to do auto-tuning, although this
> might be a hueristic that might need to be configurable for certain
> specialized workloads.  For many other workloads, the it should be
> possible to detect regular pattern of reads and/or synchronous writes,
> and if so, use a lower nr_to_write versus if there isn't many
> synchronous I/O operations happening on that particular block device.

It's not easy to get state of the art SYNC read/write busyness.
However it is possible to "feel" them through the progress of ASYNC
writes.

- setup a per-file timeout=3*HZ
- check this in write_cache_pages:

        if (half nr_to_write pages written && timeout)
                break;

In this way we back off to nr_to_write/2 if the writeback is blocked
by some busy READs.

I'd choose to implement this advanced feature some time later :)

Thanks,
Fengguang