From: Michal Hocko <email@example.com> To: Daniel Jordan <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: Re: [RFC PATCH v3 1/7] ktask: add documentation Date: Wed, 6 Dec 2017 15:35:09 +0100 [thread overview] Message-ID: <20171206143509.GG7515@dhcp22.suse.cz> (raw) In-Reply-To: <email@example.com> Please note that I haven't checked any code in this patch series. I've just started here to see how the thing is supposed to work and what is the overall design On Tue 05-12-17 14:52:14, Daniel Jordan wrote: [...] > +Resource Limits and Auto-Tuning > +=============================== > + > +ktask has resource limits on the number of workqueue items it queues. In > +ktask, a workqueue item is a thread that runs chunks of the task until the task > +is finished. > + > +These limits support the different ways ktask uses workqueues: > + - ktask_run to run threads on the calling thread's node. > + - ktask_run_numa to run threads on the node(s) specified. > + - ktask_run_numa with nid=NUMA_NO_NODE to run threads on any node in the > + system. > + > +To support these different ways of queueing work while maintaining an efficient > +concurrency level, we need both system-wide and per-node limits on the number > +of threads. Without per-node limits, a node might become oversubscribed > +despite ktask staying within the system-wide limit, and without a system-wide > +limit, we can't properly account for work that can run on any node. > + > +The system-wide limit is based on the total number of CPUs, and the per-node > +limit on the CPU count for each node. A per-node work item counts against the > +system-wide limit. Workqueue's max_active can't accommodate both types of > +limit, no matter how many workqueues are used, so ktask implements its own. > + > +If a per-node limit is reached, the work item is allowed to run anywhere on the > +machine to avoid overwhelming the node. If the global limit is also reached, > +ktask won't queue additional work items until we fall below the limit again. > + > +These limits apply only to workqueue items--that is, additional threads beyond > +the one starting the task. That way, one thread per task is always allowed to > +run. > + > +Within the resource limits, ktask uses a default maximum number of threads per > +task to avoid disturbing other processes on the system. Callers can change the > +limit with ktask_ctl_set_max_threads. For example, this might be used to raise > +the maximum number of threads for a boot-time initialization task when more > +CPUs than usual are idle. The last time something like this (maybe even this specific approach - I do not remember) the main objection was the auto-tuning. Unless I've missed anything here all the tuning is based on counters rather than the _current_ system utilization. There is also no mention about other characteristics (e.g. power management), resource isolation etc. So let me ask again. How do you control that the parallelized operation doesn't run outside of the limit imposed to the calling context? How do you control whether a larger number of workers should be fired when the system is idle but we want to keep many cpus idle due to power constrains. How do you control how many workers are fired based on cpu utilization? Do you talk to the scheduler to see overall/per node utilization. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to firstname.lastname@example.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"email@example.com"> firstname.lastname@example.org </a>
next prev parent reply other threads:[~2017-12-06 14:35 UTC|newest] Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-12-05 19:52 [RFC PATCH v3 0/7] ktask: multithread CPU-intensive kernel work Daniel Jordan 2017-12-05 19:52 ` [RFC PATCH v3 1/7] ktask: add documentation Daniel Jordan 2017-12-05 20:59 ` Daniel Jordan 2017-12-06 14:35 ` Michal Hocko [this message] 2017-12-06 20:32 ` Daniel Jordan 2017-12-08 12:43 ` Michal Hocko 2017-12-08 13:46 ` Daniel Jordan 2017-12-05 19:52 ` [RFC PATCH v3 2/7] ktask: multithread CPU-intensive kernel work Daniel Jordan 2017-12-05 22:21 ` Andrew Morton 2017-12-06 14:21 ` Daniel Jordan 2017-12-05 19:52 ` [RFC PATCH v3 3/7] ktask: add /proc/sys/debug/ktask_max_threads Daniel Jordan 2017-12-05 19:52 ` [RFC PATCH v3 4/7] mm: enlarge type of offset argument in mem_map_offset and mem_map_next Daniel Jordan 2017-12-05 19:52 ` [RFC PATCH v3 5/7] mm: parallelize clear_gigantic_page Daniel Jordan 2017-12-05 19:52 ` [RFC PATCH v3 6/7] hugetlbfs: parallelize hugetlbfs_fallocate with ktask Daniel Jordan 2017-12-05 19:52 ` [RFC PATCH v3 7/7] mm: parallelize deferred struct page initialization within each node Daniel Jordan 2017-12-05 22:23 ` [RFC PATCH v3 0/7] ktask: multithread CPU-intensive kernel work Andrew Morton 2017-12-06 14:21 ` Daniel Jordan
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