Re: [PATCH v3 1/2] writeback: add dirty_background_centisecs perbdi variable

[Jan Kara]

On Thu 27-09-12 15:00:18, Namjae Jeon wrote:
> 2012/9/27, Jan Kara <jack@xxxxxxx>:
> > On Thu 27-09-12 00:56:02, Wu Fengguang wrote:
> >> On Tue, Sep 25, 2012 at 12:23:06AM +0200, Jan Kara wrote:
> >> > On Thu 20-09-12 16:44:22, Wu Fengguang wrote:
> >> > > On Sun, Sep 16, 2012 at 08:25:42AM -0400, Namjae Jeon wrote:
> >> > > > From: Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
> >> > > >
> >> > > > This patch is based on suggestion by Wu Fengguang:
> >> > > > https://lkml.org/lkml/2011/8/19/19
> >> > > >
> >> > > > kernel has mechanism to do writeback as per dirty_ratio and
> >> > > > dirty_background
> >> > > > ratio. It also maintains per task dirty rate limit to keep balance
> >> > > > of
> >> > > > dirty pages at any given instance by doing bdi bandwidth
> >> > > > estimation.
> >> > > >
> >> > > > Kernel also has max_ratio/min_ratio tunables to specify percentage
> >> > > > of
> >> > > > writecache to control per bdi dirty limits and task throttling.
> >> > > >
> >> > > > However, there might be a usecase where user wants a per bdi
> >> > > > writeback tuning
> >> > > > parameter to flush dirty data once per bdi dirty data reach a
> >> > > > threshold
> >> > > > especially at NFS server.
> >> > > >
> >> > > > dirty_background_centisecs provides an interface where user can
> >> > > > tune
> >> > > > background writeback start threshold using
> >> > > > /sys/block/sda/bdi/dirty_background_centisecs
> >> > > >
> >> > > > dirty_background_centisecs is used alongwith average bdi write
> >> > > > bandwidth
> >> > > > estimation to start background writeback.
> >> >   The functionality you describe, i.e. start flushing bdi when there's
> >> > reasonable amount of dirty data on it, looks sensible and useful.
> >> > However
> >> > I'm not so sure whether the interface you propose is the right one.
> >> > Traditionally, we allow user to set amount of dirty data (either in
> >> > bytes
> >> > or percentage of memory) when background writeback should start. You
> >> > propose setting the amount of data in centisecs-to-write. Why that
> >> > difference? Also this interface ties our throughput estimation code
> >> > (which
> >> > is an implementation detail of current dirty throttling) with the
> >> > userspace
> >> > API. So we'd have to maintain the estimation code forever, possibly
> >> > also
> >> > face problems when we change the estimation code (and thus estimates in
> >> > some cases) and users will complain that the values they set originally
> >> > no
> >> > longer work as they used to.
> >>
> >> Yes, that bandwidth estimation is not all that (and in theory cannot
> >> be made) reliable which may be a surprise to the user. Which make the
> >> interface flaky.
> >>
> >> > Also, as with each knob, there's a problem how to properly set its
> >> > value?
> >> > Most admins won't know about the knob and so won't touch it. Others
> >> > might
> >> > know about the knob but will have hard time figuring out what value
> >> > should
> >> > they set. So if there's a new knob, it should have a sensible initial
> >> > value. And since this feature looks like a useful one, it shouldn't be
> >> > zero.
> >>
> >> Agreed in principle. There seems be no reasonable defaults for the
> >> centisecs-to-write interface, mainly due to its inaccurate nature,
> >> especially the initial value may be wildly wrong on fresh system
> >> bootup. This is also true for your proposed interfaces, see below.
> >>
> >> > So my personal preference would be to have bdi->dirty_background_ratio
> >> > and
> >> > bdi->dirty_background_bytes and start background writeback whenever
> >> > one of global background limit and per-bdi background limit is exceeded.
> >> > I
> >> > think this interface will do the job as well and it's easier to maintain
> >> > in
> >> > future.
> >>
> >> bdi->dirty_background_ratio, if I understand its semantics right, is
> >> unfortunately flaky in the same principle as centisecs-to-write,
> >> because it relies on the (implicitly estimation of) writeout
> >> proportions. The writeout proportions for each bdi starts with 0,
> >> which is even worse than the 100MB/s initial value for
> >> bdi->write_bandwidth and will trigger background writeback on the
> >> first write.
> >   Well, I meant bdi->dirty_backround_ratio wouldn't use writeout proportion
> > estimates at all. Limit would be
> >   dirtiable_memory * bdi->dirty_backround_ratio.
> >
> > After all we want to start writeout to bdi when we have enough pages to
> > reasonably load the device for a while which has nothing to do with how
> > much is written to this device as compared to other devices.
> >
> > OTOH I'm not particularly attached to this interface. Especially since on a
> > lot of today's machines, 1% is rather big so people might often end up
> > using dirty_background_bytes anyway.
> >
> >> bdi->dirty_background_bytes is, however, reliable, and gives users
> >> total control. If we export this interface alone, I'd imagine users
> >> who want to control centisecs-to-write could run a simple script to
> >> periodically get the write bandwith value out of the existing bdi
> >> interface and echo it into bdi->dirty_background_bytes. Which makes
> >> simple yet good enough centisecs-to-write controlling.
> >>
> >> So what do you think about exporting a really dumb
> >> bdi->dirty_background_bytes, which will effectively give smart users
> >> the freedom to do smart control over per-bdi background writeback
> >> threshold? The users are offered the freedom to do his own bandwidth
> >> estimation and choose not to rely on the kernel estimation, which will
> >> free us from the burden of maintaining a flaky interface as well. :)
> >   That's fine with me. Just it would be nice if we gave
> > bdi->dirty_background_bytes some useful initial value. Maybe like
> > dirtiable_memory * dirty_background_ratio?
> Global dirty_background_bytes default value is zero that means
> flushing is started based on dirty_background_ratio and dirtiable
> memory.
> Is it correct to set per bdi default dirty threshold
> (bdi->dirty_background_bytes) equal to global dirty threshold  -
> dirtiable_memory * dirty_background_ratio ?
  Right, the default setting I proposed doesn't make a difference. And it's
not obvious how to create one which is more meaningful. Pity.

> In my opinion, default setting for per bdi-> dirty_background_bytes
> should be zero to avoid any confusion and any change in default
> writeback behaviour.
  OK, fine with me.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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