Hi, On Mon, Dec 12, 2016 at 4:08 PM, Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > OK, so I just put a printk in wait_iff_congested() and it didn't show > me waiting for the timeout (!). I know that I saw > wait_iff_congested() in the originally reproduction of this problem, > but it appears that in my little "balloon" reproduction it's not > actually involved... > > > ...I dug further and it appears that __alloc_pages_direct_reclaim() is > actually what's slow. Specifically it looks as if shrink_zone() can > actually take quite a while. As I've said, I'm not an expert on the > memory manager but I'm not convinced that it's wrong for the direct > reclaim path to be pretty slow at times, especially when I'm putting > an abnormally high amount of stress on it. > > I'm going to take this as further evidence that the patch being > discussed in this thread is a good one (AKA don't hold the dm bufio > lock while allocating memory). :) If it's unexpected that > shrink_zone() might take several seconds when under extreme memory > pressure then I can do some additional digging. Do note that I am > running with "zram" and remember that I'm on an ancient 4.4-based > kernel, so perhaps one of those two factors causes problems. Sadly, I couldn't get this go as just "the way things were" in case there was some major speedup to be had here. :-P I tracked this down to shrink_list() taking 1 ms per call (perhaps because I have HZ=1000?) and in shrink_lruvec() the outer loop ran many thousands of times. Thus the total time taken by shrink_lruvec() could easily be many seconds. Wow, interesting, when I change HZ to 100 instead of 1000 then the behavior changes quite a bit. I can still get my bufio lock warning easily, but all of a sudden shrink_lruvec() isn't slow. :-P OK, really truly going to stop digging further now... ;) Presumably reporting weird behaviors with old kernels doesn't help anyone in mainline, and I can buy the whole "memory accesses are slow when you start thrashing the system" argument. -Doug -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
