Messages in this thread |  | | Date | Fri, 10 Nov 2000 08:34:14 +0100 (CET) | From | Mike Galbraith <> | Subject | Re: [BUG] /proc/<pid>/stat access stalls badly for swapping process, 2.4.0-test10 |
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On Thu, 9 Nov 2000, Linus Torvalds wrote:
> > > As to the real reason for stalls on /proc/<pid>/stat, I bet it has nothing > to do with IO except indirectly (the IO is necessary to trigger the > problem, but the _reason_ for the problem lies elsewhere). > > And it has everything to do with the fact that the way Linux semaphores > are implemented, a non-blocking process has a HUGE advantage over a > blocking one. Linux kernel semaphores are extreme unfair in that way. > > What happens is that some process is getting a lot of VM faults and gets > its VM semaphore. No contention yet. it holds the semaphore over the > IO, and now another process does a "ps". > > The "ps" process goes to sleep on the semaphore. So far so good. > > The original process releases the semaphore, which increments the count, > and wakes up the process waiting for it. Note that it _wakes_ it, it does > not give the semaphore to it. Big difference. > > The process that got woken up will run eventually. Probably not all that > immediately, because the process that woke it (and held the semaphore) > just slept on a page fault too, so it's not likely to immediately > relinquish the CPU. > > The original running process comes back faulting again, finds the > semaphore still unlocked (the "ps" process is awake but has not gotten to > run yet), gets the semaphore, and falls asleep on the IO for the next > page. > > The "ps" process actually gets to run now, but it's a bit late. The > semaphore is locked again. > > Repeat until luck breaks the bad circle. > > (This schenario, btw, is much harder to trigger on SMP than on UP. And > it's completely separate from the issue of simple disk bandwidth issues > which can obviously cause no end of stalls on anything that needs the > disk, and which can also happen on SMP).
Unfortunately, it didn't help in the scenario I'm running.
time make -j30 bzImage:
real 14m19.987s (within stock variance) user 6m24.480s sys 1m12.970s
procs memory swap io system cpu r b w swpd free buff cache si so bi bo in cs us sy id 31 2 1 12 1432 4440 12660 0 12 27 151 202 848 89 11 0 34 4 1 1908 2584 536 5376 248 1904 602 763 785 4094 63 32 5 13 19 1 64140 67728 604 33784 106500 84612 43625 21683 19080 52168 28 22 50
I understood the above well enough to be very interested in seeing what happens with flush IO restricted.
-Mike
[try_to_free_pages()->swap_out()/shm_swap().. can fight over who gets to shrink the best candidate's footprint?]
Thanks!
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