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. 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?
