[TUHS] signals and blocked in I/O

Bakul Shah bakul at bitblocks.com
Sat Dec 2 07:33:46 AEST 2017

On Dec 1, 2017, at 9:26 AM, Larry McVoy <lm at mcvoy.com> wrote:
> On Fri, Dec 01, 2017 at 09:33:49AM -0700, Warner Losh wrote:
>>> Or what you do is kill the process, tear down all of the pages except those
>>> that are locked for I/O, leave those in the process and wait for the I/O to
>>> get done.  That might be simpler.
>> Perhaps. Even that may have issues with cleanup because you may also need
>> other pages to complete the I/O processing since I think that things like
>> aio allocate a tiny bit of memory associated with the requesting process
>> and need that memory to finish the I/O. It's certainly not a requirement
>> that all I/O initiated by userland have no extra state stored in the
>> process' address space associated with it. Since the unwinding happens at a
>> layer higher than the disk driver, who knows what those guys do, eh?
> Yeah, it's not an easy fix but the problem we are having right now is that
> the system is thrashing.  Why the OOM code isn't fixing it I don't know.
> It just feels busted.

So OOM code kills a (random) process in hopes of freeing up
some pages but if this process is stuck in diskIO, nothing
can be freed and everything grinds to a halt. Is this right?

If so, one work around is to kill a process that is *not*
sleeping at an uninterruptable priority :-)/2 This is
separate from any policy of how to choose a victim.

If the queue of dirty pages is growing longer and longer
as the page washer can't keep up, this is analogous to
the bufferbloat problem in networking. You have to test
if this is what is going on. If so, may be you can figure
out how to keep the queues short.

But before any fixes, I would strongly suggest instrumenting
the code to understand what is going on and then instrument
the code further to test out various hypotheses. Once a clear
mental model is in place, the fix will be obvious!

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