On Mon 06-04-20 11:36:01, Jan Kara wrote: > On Mon 06-04-20 09:44:53, Michal Hocko wrote: > > 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. > > Let me think out loud so see whether I understand this properly. There are > two BDIs involved in NFS loop mount - the NFS virtual BDI (let's call it > simply NFS-bdi) and the bdi of the real filesystem that is backing NFS > (let's call this real-bdi). The case we are concerned about is when NFS-bdi > is full of dirty pages so that global dirty limit of the machine is > exceeded. Then flusher thread will take dirty pages from NFS-bdi and send > them over localhost to nfsd. Nfsd, which has PF_LOCAL_THROTTLE set, will take > these pages and write them to real-bdi. Now because PF_LOCAL_THROTTLE is > set for nfsd, the fact that we are over global limit does not take effect > and nfsd is still able to write to real-bdi until dirty limit on real-bdi > is reached. So things should work as Neil writes AFAIU. Thanks for the clarification. I was not aware of the 2 bdi situation. This makes more sense now. Maybe this is a trivial fact for everybody who is more familiar with nfs internals but it would be so much esier to follow if it was explicit in the changelog. -- Michal Hocko SUSE Labs
