On 7/25/18 1:15 AM, Johannes Weiner wrote: > Hi Balbir, > > On Tue, Jul 24, 2018 at 07:14:02AM +1000, Balbir Singh wrote: >> Does the mechanism scale? I am a little concerned about how frequently >> this infrastructure is monitored/read/acted upon. > > I expect most users to poll in the frequency ballpark of the running > averages (10s, 1m, 5m). Our OOMD defaults to 5s polling of the 10s > average; we collect the 1m average once per minute from our machines > and cgroups to log the system/workload health trends in our fleet. > > Suren has been experimenting with adaptive polling down to the > millisecond range on Android. > I think this is a bad way of doing things, polling only adds to overheads, there needs to be an event driven mechanism and the selection of the events need to happen in user space. >> Why aren't existing mechanisms sufficient > > Our existing stuff gives a lot of indication when something *may* be > an issue, like the rate of page reclaim, the number of refaults, the > average number of active processes, one task waiting on a resource. > > But the real difference between an issue and a non-issue is how much > it affects your overall goal of making forward progress or reacting to > a request in time. And that's the only thing users really care > about. It doesn't matter whether my system is doing 2314 or 6723 page > refaults per minute, or scanned 8495 pages recently. I need to know > whether I'm losing 1% or 20% of my time on overcommitted memory. > > Delayacct is time-based, so it's a step in the right direction, but it > doesn't aggregate tasks and CPUs into compound productivity states to > tell you if only parts of your workload are seeing delays (which is > often tolerable for the purpose of ensuring maximum HW utilization) or > your system overall is not making forward progress. That aggregation > isn't something you can do in userspace with polled delayacct data. By aggregation you mean cgroup aggregation? > >> -- why is the avg delay calculation in the kernel? > > For one, as per above, most users will probably be using the standard > averaging windows, and we already have this highly optimizd > infrastructure from the load average. I don't see why we shouldn't use > that instead of exporting an obscure number that requires most users > to have an additional library or copy-paste the loadavg code. > > I also mentioned the OOM killer as a likely in-kernel user of the > pressure percentages to protect from memory livelocks out of the box, > in which case we have to do this calculation in the kernel anyway. > >> There is no talk about the overhead this introduces in general, may be >> the details are in the patches. I'll read through them > > I sent an email on benchmarks and overhead in one of the subthreads, I > will include that information in the cover letter in v3. > > https://lore.kernel.org/lkml/20180718215644.GB2838@xxxxxxxxxxx/ Thanks, I'll take a look Balbir Singh.
