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SubjectRe: [BUG] /proc/<pid>/stat access stalls badly for swapping process, 2.4.0-test10
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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