Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Michal Hocko]

On Sat 04-04-20 08:40:17, Neil Brown wrote:
> On Fri, Apr 03 2020, Michal Hocko wrote:
> 
> > On Thu 02-04-20 10:53:20, Neil Brown wrote:
> >> 
> >> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> >> loop block driver, where a daemon needs to write to one bdi in
> >> order to free up writes queued to another bdi.
> >> 
> >> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> >> pages, so that it can still dirty pages after other processses have been
> >> throttled.
> >> 
> >> This approach was designed when all threads were blocked equally,
> >> independently on which device they were writing to, or how fast it was.
> >> Since that time the writeback algorithm has changed substantially with
> >> different threads getting different allowances based on non-trivial
> >> heuristics.  This means the simple "add 25%" heuristic is no longer
> >> reliable.
> >> 
> >> This patch changes the heuristic to ignore the global limits and
> >> consider only the limit relevant to the bdi being written to.  This
> >> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> >> should not introduce surprises.  This has the desired result of
> >> protecting the task from the consequences of large amounts of dirty data
> >> queued for other devices.
> >
> > While I understand that you want to have per bdi throttling for those
> > "special" files I am still missing how this is going to provide the
> > additional room that the additnal 25% gave them previously. I might
> > misremember or things have changed (what you mention as non-trivial
> > heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
> > forward progress. Care to expan some more on how this is handled now?
> > Maybe we do not need it anymore but calling that out explicitly would be
> > really helpful.
> 
> The 25% was a means to an end, not an end in itself.
> 
> The problem is that the NFS server needs to be able to write to the
> backing filesystem when the dirty memory limits have been reached by
> being totally consumed by dirty pages on the NFS filesystem.
> 
> The 25% was just a way of giving an allowance of dirty pages to nfsd
> that could not be consumed by processes writing to an NFS filesystem.
> i.e. it doesn't need 25% MORE, it needs 25% PRIVATELY.  Actually it only
> really needs 1 page privately, but a few pages give better throughput
> and 25% seemed like a good idea at the time.

Yes this part is clear to me.
 
> per-bdi throttling focuses on the "PRIVATELY" (the important bit) and
> de-emphasises the 25% (the irrelevant detail).

It is still not clear to me how this patch is going to behave when the
global dirty throttling is essentially equal to the per-bdi - e.g. there
is only a single bdi and now the PF_LOCAL_THROTTLE process doesn't have
anything private.

-- 
Michal Hocko
SUSE Labs