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Re: [Qemu-devel] When it's okay to treat OOM as fatal?
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] When it's okay to treat OOM as fatal? |
Date: |
Thu, 18 Oct 2018 15:54:06 +0100 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
* Markus Armbruster (address@hidden) wrote:
> "Dr. David Alan Gilbert" <address@hidden> writes:
>
> > * Markus Armbruster (address@hidden) wrote:
> >> We sometimes use g_new() & friends, which abort() on OOM, and sometimes
> >> g_try_new() & friends, which can fail, and therefore require error
> >> handling.
> >>
> >> HACKING points out the difference, but is mum on when to use what:
> >>
> >> 3. Low level memory management
> >>
> >> Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign
> >> APIs is not allowed in the QEMU codebase. Instead of these routines,
> >> use the GLib memory allocation routines g_malloc/g_malloc0/g_new/
> >> g_new0/g_realloc/g_free or QEMU's
> >> qemu_memalign/qemu_blockalign/qemu_vfree
> >> APIs.
> >>
> >> Please note that g_malloc will exit on allocation failure, so there
> >> is no need to test for failure (as you would have to with malloc).
> >> Calling g_malloc with a zero size is valid and will return NULL.
> >>
> >> Prefer g_new(T, n) instead of g_malloc(sizeof(T) * n) for the following
> >> reasons:
> >>
> >> a. It catches multiplication overflowing size_t;
> >> b. It returns T * instead of void *, letting compiler catch more type
> >> errors.
> >>
> >> Declarations like T *v = g_malloc(sizeof(*v)) are acceptable, though.
> >>
> >> Memory allocated by qemu_memalign or qemu_blockalign must be freed with
> >> qemu_vfree, since breaking this will cause problems on Win32.
> >>
> >> Now, in my personal opinion, handling OOM gracefully is worth the
> >> (commonly considerable) trouble when you're coding for an Apple II or
> >> similar. Anything that pages commonly becomes unusable long before
> >> allocations fail.
> >
> > That's not always my experience; I've seen cases where you suddenly
> > allocate a load more memory and hit OOM fairly quickly on that hot
> > process. Most of the time on the desktop you're right.
> >
> >> Anything that overcommits will send you a (commonly
> >> lethal) signal instead. Anything that tries handling OOM gracefully,
> >> and manages to dodge both these bullets somehow, will commonly get it
> >> wrong and crash.
> >
> > If your qemu has maped it's main memory from hugetlbfs or similar pools
> > then we're looking at the other memory allocations; and that's a bit of
> > an interesting difference where those other allocations should be a lot
> > smaller.
> >
> >> But others are entitled to their opinions as much as I am. I just want
> >> to know what our rules are, preferably in the form of a patch to
> >> HACKING.
> >
> > My rule is to try not to break a happily running VM by some new
> > activity; I don't worry about it during startup.
> >
> > So for example, I don't like it when starting a migration, allocates
> > some more memory and kills the VM - the user had a happy stable VM
> > upto that point. Migration gets the blame at this point.
>
> I don't doubt reliable OOM handling would be nice. I do doubt it's
> practical for an application like QEMU.
Well, our use of glib certainly makes it much much harder.
I just try and make sure anywhere that I'm allocating a non-trivial
amount of memory (especially anything guest or user controlled) uses
the _try_ variants. That should keep a lot of the larger allocations.
However, it scares me that we've got things that can return big chunks
of JSON for example, and I don't think they're being careful about it.
Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
- [Qemu-devel] When it's okay to treat OOM as fatal?, Markus Armbruster, 2018/10/16
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Daniel P . Berrangé, 2018/10/16
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Dr. David Alan Gilbert, 2018/10/16
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Markus Armbruster, 2018/10/18
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?,
Dr. David Alan Gilbert <=
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Markus Armbruster, 2018/10/18
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Dr. David Alan Gilbert, 2018/10/18
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Markus Armbruster, 2018/10/19
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Dr. David Alan Gilbert, 2018/10/19
- Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Dr. David Alan Gilbert, 2018/10/22
Re: [Qemu-devel] When it's okay to treat OOM as fatal?, Stefan Hajnoczi, 2018/10/17