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Subject: Re: [OMPI devel] ptmalloc and pin down cache problems again
From: Brian W. Barrett (brbarret_at_[hidden])
Date: 2008-01-07 10:19:33


Nope, I think that's a valid approach. For some reason, I believe it
was problematic for the OpenIB guys to do that at the time we were
hacking up that code. But if it works, it sounds like a much better
approach.

When you make the change to the openib mpool, I'd also
MORECORE_CANNONT_TRIM back to 0. mvapi / openib were the only libraries
that needed the free in the deregistration callback -- GM apppeared to not
have that particular behavior. And I don't believe that anyone else
actually uses the deregistration callbacks.

Brian

On Mon, 7 Jan 2008, Gleb Natapov wrote:

> Hi Brian,
>
> I encountered problem with ptmalloc an registration cache. I see that
> you (I think it was you) disabled shrinking of a heap memory allocated
> by sbrk by setting MORECORE_CANNOT_TRIM to 1. The comment explains that
> it should be done because freeing of small objects is not reentrant so
> if ompi memory subsystem callback will call free() the code will deadlock.
> And the trick indeed works in single threaded programs, but in multithreaded
> programs ptmalloc may allocate a heap not only by sbrk, but by mmap too. This
> is called "arena". Each thread may have arenas of its own. The problem is
> that ptmalloc may free an arena by calling munmap() and then free() that
> is called from our callback deadlocks. I tried to compile with USE_ARENAS set
> to 0, but the code doesn't compile. I can fix compilation problem of
> cause, but it seems that it is not so good idea to disable this feature.
> The ptmalloc scalability depends on it (and even if we will disable it
> ptmalloc may still create arena by mmap if sbrk fails). I see only one
> way to solve this problem: to not call free() inside mpool callbacks.
> If freeing of a memory is needed (and it is needed since IB unregister
> calls free()) the works should be deferred. For IB mpool we can check what
> needs to be unregistered inside a callback, but actually call unregister()
> from next mpool->register() call. Do you see any problems with this
> approach?
>
> --
> Gleb.
>
>