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. -- Michal Hocko SUSE Labs
