Re: [RFC] writeback and cgroup

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On Fri 20-04-12 21:34:41, Wu Fengguang wrote:
> On Thu, Apr 19, 2012 at 10:26:35PM +0200, Jan Kara wrote:
> > > It's not uncommon for me to see filesystems sleep on PG_writeback
> > > pages during heavy writeback, within some lock or transaction, which in
> > > turn stall many tasks that try to do IO or merely dirty some page in
> > > memory. Random writes are especially susceptible to such stalls. The
> > > stable page feature also vastly increase the chances of stalls by
> > > locking the writeback pages. 
> > > 
> > > Page reclaim may also block on PG_writeback and/or PG_dirty pages. In
> > > the case of direct reclaim, it means blocking random tasks that are
> > > allocating memory in the system.
> > > 
> > > PG_writeback pages are much worse than PG_dirty pages in that they are
> > > not movable. This makes a big difference for high-order page allocations.
> > > To make room for a 2MB huge page, vmscan has the option to migrate
> > > PG_dirty pages, but for PG_writeback it has no better choices than to
> > > wait for IO completion.
> > > 
> > > The difficulty of THP allocation goes up *exponentially* with the
> > > number of PG_writeback pages. Assume PG_writeback pages are randomly
> > > distributed in the physical memory space. Then we have formula
> > > 
> > >         P(reclaimable for THP) = 1 - P(hit PG_writeback)^256
> >   Well, this implicitely assumes that PG_Writeback pages are scattered
> > across memory uniformly at random. I'm not sure to which extent this is
> > true...
> Yeah, when describing the problem I was also thinking about the
> possibilities of optimization (it would be a very good general
> improvements). Or maybe Mel already has some solutions :)
> > Also as a nitpick, this isn't really an exponential growth since
> > the exponent is fixed (256 - actually it should be 512, right?). It's just
> Right, 512 4k pages to form one x86_64 2MB huge pages.
> > a polynomial with a big exponent. But sure, growth in number of PG_Writeback
> > pages will cause relatively steep drop in the number of available huge
> > pages.
> It's exponential indeed, because "1 - p(x)" here means "p(!x)".
> It's exponential for a 10x increase in x resulting in 100x drop of y.
  If 'x' is the probability page has PG_Writeback set, then the probability
a huge page has a single PG_Writeback page is (as you almost correctly wrote):
(1-x)^512. This is a polynominal by the definition: It can be
expressed as $\sum_{i=0}^n a_i*x^i$ for $a_i\in R$ and $n$ finite.

The expression decreases fast as x approaches to 1, that's for sure, but
that does not make it exponential. Sorry, my mathematical part could not
resist this terminology correction.

> > ...
> > > > > To me, balance_dirty_pages() is *the* proper layer for buffered writes.
> > > > > It's always there doing 1:1 proportional throttling. Then you try to
> > > > > kick in to add *double* throttling in block/cfq layer. Now the low
> > > > > layer may enforce 10:1 throttling and push balance_dirty_pages() away
> > > > > from its balanced state, leading to large fluctuations and program
> > > > > stalls.
> > > > 
> > > > Just do the same 1:1 inside each cgroup.
> > > 
> > > Sure. But the ratio mismatch I'm talking about is inter-cgroup.
> > > For example there are only 2 dd tasks doing buffered writes in the
> > > system. Now consider the mismatch that cfq is dispatching their IO
> > > requests at 10:1 weights, while balance_dirty_pages() is throttling
> > > the dd tasks at 1:1 equal split because it's not aware of the cgroup
> > > weights.
> > > 
> > > What will happen in the end? The 1:1 ratio imposed by
> > > balance_dirty_pages() will take effect and the dd tasks will progress
> > > at the same pace. The cfq weights will be defeated because the async
> > > queue for the second dd (and cgroup) constantly runs empty.
> >   Yup. This just shows that you have to have per-cgroup dirty limits. Once
> > you have those, things start working again.
> Right. I think Tejun was more of less aware of this.
> I was rather upset by this per-memcg dirty_limit idea indeed. I never
> expect it to work well when used extensively. My plan was to set the
> default memcg dirty_limit high enough, so that it's not hit in normal.
> Then Tejun came and proposed to (mis-)use dirty_limit as the way to
> convert the dirty pages' backpressure into real dirty throttling rate.
> No, that's just crazy idea!
> Come on, let's not over-use memcg's dirty_limit. It's there as the
> *last resort* to keep dirty pages under control so as to maintain
> interactive performance inside the cgroup. However if used extensively
> in the system (like dozens of memcgs all hit their dirty limits), the
> limit itself may stall random dirtiers and create interactive
> performance issues!
> In the recent days I've come up with the idea of memcg.dirty_setpoint
> for the blkcg backpressure stuff. We can use that instead.
> memcg.dirty_setpoint will scale proportionally with blkcg.writeout_rate.
> Imagine bdi_setpoint. It's all the same concepts. Why we need this?
> Because if blkcg A and B does 10:1 weights and are both doing buffered
> writes, their dirty pages should better be maintained around 10:1
> ratio to avoid underrun and hopefully achieve better IO size.
> memcg.dirty_limit cannot guarantee that goal.
  I agree that to avoid stalls of throttled processes we shouldn't be
hitting memcg.dirty_limit on a regular basis. When I wrote we need "per
cgroup dirty limits" I actually imagined something like you write above -
do complete throttling computations within each memcg - estimate throughput
available for it, compute appropriate dirty rates for it's processes and
from its dirty limit estimate appropriate setpoint to balance around.

> But be warned! Partitioning the dirty pages always means more
> fluctuations of dirty rates (and even stalls) that's perceivable by
> the user. Which means another limiting factor for the backpressure
> based IO controller to scale well.
  Sure, the smaller the memcg gets, the more noticeable these fluctuations
would be. I would not expect memcg with 200 MB of memory to behave better
(and also not much worse) than if I have a machine with that much memory...

Jan Kara <jack@xxxxxxx>

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