Re: [PATCH v2] mm: implement write-behind policy for sequential file writes

[Konstantin Khlebnikov]

On 23/09/2019 18.36, Jens Axboe wrote:
> On 9/20/19 5:10 PM, Linus Torvalds wrote:
> > On Fri, Sep 20, 2019 at 4:05 PM Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > Now, I hear you say "those are so small these days that it doesn't
> > > matter". And maybe you're right. But particularly for slow media,
> > > triggering good streaming write behavior has been a problem in the
> > > past.
> >
> > Which reminds me: the writebehind trigger should likely be tied to the
> > estimate of the bdi write speed.
> >
> > We _do_ have that avg_write_bandwidth thing in the bdi_writeback
> > structure, it sounds like a potentially good idea to try to use that
> > to estimate when to do writebehind.
> >
> > No?
>
> I really like the feature, and agree it should be tied to the bdi write
> speed. How about just making the tunable acceptable time of write behind
> dirty? Eg if write_behind_msec is 1000, allow 1s of pending dirty before
> starting writbeack.

I haven't digged into it yet.

But IIRR writeback speed estimation has some problems:

There is no "slow start" - initial speed is 100MiB/s.
This is especially bad for slow usb disks - right after plugging
we'll accumulate too much dirty cache before starting writeback.

And I've seen problems with cgroup-writeback:
each cgroup has own estimation, doesn't work well for short-living cgroups.