On Fri 20-08-21 23:20:40, yong w wrote: > Michal Hocko <mhocko@xxxxxxxx> 于2021年8月20日周五 下午7:26写道： > > > > On Thu 19-08-21 16:53:39, yongw.pur@xxxxxxxxx wrote: > > > From: wangyong <wang.yong@xxxxxxxxxx> > > > > > > Inspired by PSI features, vmpressure inotifier function should > > > also be configured to decide whether it is used, because it is an > > > independent feature which notifies the user of memory pressure. > > > > Yes, it is an independent feature indeed but what is the actual reason > > to put a more configuration space here. Config options are not free both > > from the user experience POV as well as the code maintenance. Why do we > > need to disable this feature. Who can benefit from such a setup? > > > > > So we add configuration to control whether vmpressure notifier is > > > enabled, and provide a boot parameter to use vmpressure notifier > > > flexibly. > > > > Flexibility is nice but not free as mentioned above. > > > > > Use Christoph Lamenter’s pagefault tool > > > (https://lkml.org/lkml/2006/8/29/294) for comparative testing. > > > Test with 5.14.0-rc5-next-20210813 on x86_64 4G Ram > > > To ensure that the vmpressure function is executed, we enable zram > > > and let the program occupy memory so that some memory is swapped out > > > > > > unpatched: > > > Gb Rep Thr CLine User(s) System(s) Wall(s) flt/cpu/s fault/wsec > > > 2 1 1 1 0.1 0.97 1.13 485490.062 463533.34 > > > 2 1 1 1 0.11 0.96 1.12 483086.072 465309.495 > > > 2 1 1 1 0.1 0.95 1.11 496687.098 469887.643 > > > 2 1 1 1 0.09 0.97 1.11 489711.434 468402.102 > > > 2 1 1 1 0.13 0.94 1.12 484159.415 466080.941 > > > average 0.106 0.958 1.118 487826.8162 466642.7042 > > > > > > patched and CONFIG_MEMCG_VMPRESSURE is not set: > > > Gb Rep Thr CLine User(s) System(s) Wall(s) flt/cpu/s fault/wsec > > > 2 1 1 1 0.1 0.96 1.1 490942.682 473125.98 > > > 2 1 1 1 0.08 0.99 1.13 484987.521 463161.975 > > > 2 1 1 1 0.09 0.96 1.09 498824.98 476696.066 > > > 2 1 1 1 0.1 0.97 1.12 484127.673 465951.238 > > > 2 1 1 1 0.1 0.97 1.11 487032 468964.662 > > > average 0.094 0.97 1.11 489182.9712 469579.9842 > > > > > > According to flt/cpu/s, performance improved by 0.2% which is not obvious. > > > > I haven't checked how are those numbers calculated but from a very brief > > look it seems like the variation between different runs is higher than > > 0.2%. Have you checked the average against standard deviation to get a > > better idea whether the difference is really outside of the noise? > > -- > > Michal Hocko > > SUSE Labs > > Thanks for your reply. > The reason for adding configuration is as follows： All those reasons should be a part of the changelog. > 1. Referring to [PATCH] psi: make disabling/enabling easier for vendor > kernels, the modification > is also applicable to vmpressure. > > 2. With the introduction of psi into the kernel, there are two memory > pressure monitoring methods， > it is not necessary to use both and it makes sense to make vmpressure > configurable. I am not sure these are sufficient justifications but that is something to discuss. And hence it should be a part of the changelog. > 3. In the case where the user does not need vmpressure, vmpressure > calculation is additional overhead. You should quantify that and argue why that overhead cannot be further reduced without config/boot time knobs. > In some special scenes with tight memory, vmpressure will be executed > frequently.we use "likely" and "inline" > to improve the performance of the kernel, why not reduce some > unnecessary calculations? I am all for improving the code. Is it possible to do it by other means? E.g. reduce a potential overhead when there no events registered? -- Michal Hocko SUSE Labs
