On Mon 18-09-17 10:44:52, kemi wrote: > > > On 2017年09月15日 22:28, Michal Hocko wrote: > > On Fri 15-09-17 07:16:23, Dave Hansen wrote: > >> On 09/15/2017 04:49 AM, Michal Hocko wrote: > >>> Why do we need an auto-mode? Is it safe to enforce by default. > >> > >> Do we *need* it? Not really. > >> > >> But, it does offer the best of both worlds: The vast majority of users > >> see virtually no impact from the counters. The minority that do need > >> them pay the cost *and* don't have to change their tooling at all. > > > > Just to make it clear, I am not really opposing. It just adds some code > > which we can safe... It is also rather chatty for something that can be > > true/false. > > > > It has benefit, as Dave mentioned above. > Actually, it adds some coding complexity to provide a tuning interface with > on/off/auto mode. Using human-readable string instead of magic number makes > it easier to use, people probably don't need to review the ABI doc again > before using it. So, I don't think that should be a problem Is this a thing that would be changed very often. I suspect that once needed it will be set in a startup sysctl configuration and there will be no further need to touch it again. > >>> Is it> possible that userspace can get confused to see 0 NUMA stats in > >> the > >>> first read while other allocation stats are non-zero? > >> > >> I doubt it. Those counters are pretty worthless by themselves. I have > >> tooling that goes and reads them, but it aways displays deltas. Read > >> stats, sleep one second, read again, print the difference. > > > > This is how I use them as well. > > > >> The only scenario I can see mattering is someone who is seeing a > >> performance issue due to NUMA allocation misses (or whatever) and wants > >> to go look *back* in the past. > > > > yes > > > > If it really matters, setting vmstat_mode=strict as a default option is a simple > way to fix it. What's your idea? thanks Well, we are usually very conservative when changing the default behavior. The primary reason why I was asking is that the auto mode doesn't make much sense unless it is the default. I fully realize that such an hypothetical breakage is really hard to envision but considering it is more code to allow auto mode than a simple on/off (we have parsing helpers for that AFAIR) then I would rather go with the simpler option. This is up to you of course. -- Michal Hocko SUSE Labs
