Messages in this thread | | | Subject | Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression | From | Yang Shi <> | Date | Mon, 5 Nov 2018 12:17:59 -0800 |
| |
On 11/5/18 10:35 AM, Linus Torvalds wrote: > On Mon, Nov 5, 2018 at 10:28 AM Yang Shi <yang.shi@linux.alibaba.com> wrote: >> Actually, the commit is mainly for optimizing the long stall time caused >> by holding mmap_sem by write when unmapping or shrinking large mapping. >> It downgrades write mmap_sem to read when zapping pages. So, it looks >> the downgrade incurs more context switches. This is kind of expected. >> >> However, the test looks just shrink the mapping with one normal 4K page >> size. It sounds the overhead of context switches outpace the gain in >> this case at the first glance. > I'm not seeing why there should be a context switch in the first place. > > Even if you have lots of concurrent brk() users, they should all block > exactly the same way as before (a write lock blocks against a write > lock, but it *also* blocks against a downgraded read lock).
Yes, it is true. The brk() users will not get waken up. What I can think of for now is there might be other helper processes and/or kernel threads are waiting for read mmap_sem. They might get waken up by the downgrade.
But, I also saw huge increase in cpu idle time and sched_goidle events. Not have clue yet for why idle goes up.
20610709 ± 15% +2376.0% 5.103e+08 ± 34% cpuidle.C1.time 28753819 ± 39% +1054.5% 3.319e+08 ± 49% cpuidle.C3.time
175049 ± 72% +840.7% 1646720 ± 72% sched_debug.cpu.sched_goidle.stddev
Thanks, Yang
> > So no, I don't want just some limit to hide this problem for that > particular test. There's something else going on. > > Linus
| |