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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: |
Tue, 16 Oct 2018 14:33:41 +0100 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
* 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.
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 <=
- 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, 2018/10/18
- 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