On 8/14/24 10:15 AM, Wen Yang wrote: > > > On 2024/8/11 18:26, Mateusz Guzik wrote: >> On Sun, Aug 11, 2024 at 04:59:54PM +0800, Wen Yang wrote: >>> For the NON-SEMAPHORE eventfd, a write (2) call adds the 8-byte integer >>> value provided in its buffer to the counter, while a read (2) returns the >>> 8-byte value containing the value and resetting the counter value to 0. >>> Therefore, the accumulated value of multiple writes can be retrieved by a >>> single read. >>> >>> However, the current situation is to immediately wake up the read thread >>> after writing the NON-SEMAPHORE eventfd, which increases unnecessary CPU >>> overhead. By introducing a configurable rate limiting mechanism in >>> eventfd_write, these unnecessary wake-up operations are reduced. >>> >>> >> [snip] >> >>> # ./a.out -p 2 -s 3 >>> The original cpu usage is as follows: >>> 09:53:38 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle >>> 09:53:40 PM 2 47.26 0.00 52.74 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> 09:53:40 PM 3 44.72 0.00 55.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> >>> 09:53:40 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle >>> 09:53:42 PM 2 45.73 0.00 54.27 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> 09:53:42 PM 3 46.00 0.00 54.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> >>> 09:53:42 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle >>> 09:53:44 PM 2 48.00 0.00 52.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> 09:53:44 PM 3 45.50 0.00 54.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> >>> Then enable the ratelimited wakeup, eg: >>> # ./a.out -p 2 -s 3 -r1000 -c2 >>> >>> Observing a decrease of over 20% in CPU utilization (CPU # 3, 54% ->30%), as shown below: >>> 10:02:32 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle >>> 10:02:34 PM 2 53.00 0.00 47.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> 10:02:34 PM 3 30.81 0.00 30.81 0.00 0.00 0.00 0.00 0.00 0.00 38.38 >>> >>> 10:02:34 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle >>> 10:02:36 PM 2 48.50 0.00 51.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> 10:02:36 PM 3 30.20 0.00 30.69 0.00 0.00 0.00 0.00 0.00 0.00 39.11 >>> >>> 10:02:36 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle >>> 10:02:38 PM 2 45.00 0.00 55.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 >>> 10:02:38 PM 3 27.08 0.00 30.21 0.00 0.00 0.00 0.00 0.00 0.00 42.71 >>> >>> >> >> Where are these stats from? Is this from your actual program you coded >> the feature for? >> >> The program you inlined here does next to nothing in userspace and >> unsurprisingly the entire thing is dominated by kernel time, regardless >> of what event rate can be achieved. >> >> For example I got: /a.out -p 2 -s 3 5.34s user 60.85s system 99% cpu 66.19s (1:06.19) total >> >> Even so, looking at perf top shows me that a significant chunk is >> contention stemming from calls to poll -- perhaps the overhead will >> sufficiently go down if you epoll instead? > > We have two threads here, one publishing and one subscribing, running > on CPUs 2 and 3 respectively. If we further refine and collect > performance data on CPU 2, we will find that a large amount of CPU is > consumed on the spin lock of the wake-up logic of event write, for > example: This is hardly surprising - you've got probably the worst kind of producer/consumer setup here, with the producer on one CPU, and the consumer on another. You force this relationship by pinning both of them. Then you have a queue in between, and locking that needs to be acquired on both sides. It's hard to come up with a WORSE way of doing that. I'll have to agree with the notion that you're using the wrong tool for the job, and hacking around it is not the right solution. -- Jens Axboe
