On Wed, Sep 23, 2009 at 10:08:40PM +0800, Chris Mason wrote:
> On Wed, Sep 23, 2009 at 09:05:41AM +0800, Wu Fengguang wrote:
>
> [ timeslice based limits on number of pages sent by the bdi threads ]
>
> > >
> > > The reason I prefer the timeslice idea is that we don't need the
> > > hardware to tell us how fast it is. We just write for a while and move
> > > on.
> >
> > That makes sense. Note that the triple (pages, page segments,
> > submission time) can somehow adapt to hardware capabilities
> > (and at least won't hurt fast arrays).
> >
> > - max pages are set to large enough number for big arrays
> > - max page segments could be based on the existing blk_queue_nonrot()
> > - submission time = 1s, which is mainly a safeguard for slow devices
> > (ie. usb stick), to prevent one single inode from taking too much
> > time. This time limit has little performance impacts.
> >
> > Possible merits are
> > - these parameters are concrete ones and easy to handle
> > - it's natural to implement related logics in the VFS level
> > - file systems can do nothing to get most benefits
> >
> > Also the (now necessary) per-invocation limit could be somehow
> > eliminated when balance_dirty_pages() does not do IO itself.
>
> I think there are probably a lot of good ways to improve on our single
> max number of pages metric from today
Yes, as always, it benefits to work out some prototype solutions for
evaluation and comparison.
> , but I'm worried about the
> calculation time finding page segments. The radix tree
> isn't all that well suited to it.
I didn't mean to "calculate" the page segments. But rather to do this
in write_cache_pages:
if (this page index is 1MB away from prev page index)
wbc->page_segments--;
> But, if you've got a patch I'd be happy to run a comparison against it.
> Jens' box will be better at showing any CPU cost to the radix walking.
Thanks!
Regards,
Fengguang
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