On Tue, 29 Oct 2002, Rusty Russell wrote:
> >
> > No bound to the number of possible OOM kills, but what problem is that?
>
> Sorry, I'm obviously not making myself clear, since I've said this
> three times now.
>
> 1) The memory is required for one whole RCU period (whether from
> kmalloc or the mempool). This can be an almost arbitrarily long
> time (I've seen it take a good fraction of a second).
That's a very short time compared with an OOMing thrash: no worries there.
> 2) This is a problem, because other tasks could be OOM killed during
> that period, and could also try to use this mempool.
They'll try to use the mempool, maybe some will be allowed to wait
for their kmalloc(GFP_KERNEL) memory, and others will be PF_MEMDIEd and
proceed to take a reserved mempool buffer, and others will be PF_MEMDIEd
and have to wait for a reserved mempool buffer. Which will be released
to them in due course. No worries there.
> 3) So, the size of the mempool which guarantees there will be enough?
> It's equal to the number of things you might free, which means
> you might as well allocate them together.
No, they take their turns. It's sure not as efficient as each getting
a kmalloc'ed buffer immediately, but its failures will be rare. And
it doesn't matter whether the failures only occur when heading for
OOM-kill or not: we just don't want failure to be the common case.
If kmalloc evolves into something that normally fails half the time,
well, I'd think that'd be called a bug.
> This is the correctness problem with the mempool IPC implementation.
No. There may be other situations which might need at least
NR_CPUS reserved mempool buffer to avoid deadlock, but that's not
the case here. Looks like mempool will be superseded as you wish
in the IPC context, fine; but I do think you need to take a look
at mempool_alloc: it's a different beast from what you suppose.
Hugh
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This archive was generated by hypermail 2b29 : Thu Oct 31 2002 - 22:00:40 EST