On Mon, Feb 21, 2022 at 06:47:25PM -0700, Yu Zhao wrote: > On Mon, Feb 21, 2022 at 2:02 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote: > > > > On Tue, Feb 15, 2022 at 08:22:10PM -0700, Yu Zhao wrote: > > > > Please consider splitting "enable" and "features" attributes. > > > > > > How about s/Features/Components/? > > > > I meant to use two attributes: > > > > /sys/kernel/mm/lru_gen/enable for the main breaker, and > > /sys/kernel/mm/lru_gen/features (or components) for the branch breakers > > It's a bit superfluous for my taste. I generally consider multiple > items to fall into the same category if they can be expressed by a > type of array, and I usually pack an array into a single file. > > From your last review, I gauged this would be too overloaded for your > taste. So I'd be happy to make the change if you think two files look > more intuitive from user's perspective. I do think that two attributes are more user-friendly, but I don't feel strongly about it. > > > > As for the descriptions, what is the user-visible effect of these features? > > > > How different modes of clearing the access bit are reflected in, say, GUI > > > > responsiveness, database TPS, or probability of OOM? > > > > > > These remain to be seen :) I just added these switches in v7, per Mel's > > > request from the meeting we had. These were never tested in the field. > > > > I see :) > > > > It would be nice to have a description or/and examples of user-visible > > effects when there will be some insight on what these features do. > > How does the following sound? > > Clearing the accessed bit in large batches can theoretically cause > lock contention (mmap_lock), and if it happens the 0x0002 switch can > disable this feature. In this case the multigenerational LRU suffers a > minor performance degradation. > Clearing the accessed bit in non-leaf page table entries was only > verified on Intel and AMD, and if it causes problems on other x86 > varieties the 0x0004 switch can disable this feature. In this case the > multigenerational LRU suffers a negligible performance degradation. LGTM > > > > > +:Debugfs interface: ``/sys/kernel/debug/lru_gen`` has the following > > > > > > > > Is debugfs interface relevant only for datacenters? > > > > > > For the moment, yes. > > > > And what will happen if somebody uses these interfaces outside > > datacenters? As soon as there is a sysfs intefrace, somebody will surely > > play with it. > > > > I think the job schedulers might be the most important user of that > > interface, but the documentation should not presume it is the only user. > > Other ideas are more like brainstorming than concrete use cases, e.g., > for desktop users, these interface can in theory speed up hibernation > (suspend to disk); for VM users, they can again in theory support auto > ballooning. These niches are really minor and less explored compared > with the data center use cases which have been dominant. > > I was hoping we could focus on the essential and take one step at a > time. Later on, if there is additional demand and resource, then we > expand to cover more use cases. Apparently I was not clear :) I didn't mean that you should describe other use-cases, I rather suggested to make the documentation more neutral, e.g. using "a user writes to this file ..." instead of "job scheduler writes to a file ...". Or maybe add a sentence in the beginning of the "Data centers" section, for instance: Data centers ------------ + A representative example of multigenerational LRU users are job schedulers. Data centers want to optimize job scheduling (bin packing) to improve memory utilizations. Job schedulers need to estimate whether a server -- Sincerely yours, Mike.
