On 04/19/2018 03:43 PM, Andrew Morton wrote: > On Thu, 19 Apr 2018 13:09:29 -0400 Waiman Long <long...@redhat.com> wrote: > >> It was found that reading /proc/stat could be time consuming on >> systems with a lot of irqs. For example, reading /proc/stat in a >> certain 2-socket Skylake server took about 4.6ms because it had over >> 5k irqs. In that particular case, the majority of the CPU cycles for >> reading /proc/stat was spent in the kstat_irqs() function. Therefore, >> application performance can be impacted if the application reads >> /proc/stat rather frequently. >> >> The "intr" line within /proc/stat contains a sum total of all the irqs >> that have happened followed by a list of irq counts for each individual >> irq number. In many cases, the first number is good enough. The >> individual irq counts may not provide that much more information. >> >> In order to avoid this kind of performance issue, all these individual >> irq counts are now separated into a new /proc/stat_irqs file. The >> sum total irq count will stay in /proc/stat and be duplicated in >> /proc/stat_irqs. Applications that need to look up individual irq counts >> will now have to look into /proc/stat_irqs instead of /proc/stat. >> > (cc /proc maintainer) > > It's a non-backward-compatible change. For something which has > existing for so long, it would be a mighty task to demonstrate that no > existing userspace will be disrupted by this change. > > So we need to think again. A new interface which omits the per-IRQ > stats might be acceptable.
OK, how about a new /proc/stat2 file that is the same as /proc/stat except that it omits per-IRQ stats. BTW, do you have any suggestion for a better name? > Or, conceivably, a new /proc knob which disables the per-IRQ stats in > /proc/stat. That would allow operators to opt in to this disabling and > would avoid the need to alter > whatever-application-it-is-that-is-having-trouble. This seems a bit ugly > though. > > Also, the changelog is rather vague. "application performance can be > impacted". Well, *are* applications impacted? What is the real-world > performance gain which this change provides, in a real-world workload? Will clarify that in the next version. -Longman