Re: [PATCH v2 06/12] accel/tcg: better handle memory constrained systems

[Alex Bennée]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Wed, Jul 22, 2020 at 12:02:59PM -0700, Richard Henderson wrote:
>> On 7/22/20 9:44 AM, Daniel P. Berrangé wrote:
>> > OpenStack uses TCG in alot of their CI infrastructure for example
>> > and runs multiple VMs. If there's 4 VMs, that's another 4 GB of
>> > RAM usage just silently added on top of the explicit -m value.
>> > 
>> > I wouldn't be surprised if this pushes CI into OOM, even without
>> > containers or cgroups being involved, as they have plenty of other
>> > services consuming RAM in the CI VMs.
>> 
>> I would hope that CI would also supply a -tb_size to go along with that -m
>> value.  Because we really can't guess on their behalf.
>
> I've never even seen mention of -tb_size argument before myself, nor
> seen anyone else using it and libvirt doesn't set it, so I think
> this is not a valid assumption.
>
>
>> > The commit 600e17b261555c56a048781b8dd5ba3985650013 talks about this
>> > minimizing codegen cache flushes, but doesn't mention the real world
>> > performance impact of eliminating those flushes ?
>> 
>> Somewhere on the mailing list was this info.  It was so dreadfully slow it 
>> was
>> *really* noticable.  Timeouts everywhere.
>> 
>> > Presumably this makes the guest OS boot faster, but what's the before
>> > and after time ?  And what's the time like for values in between the
>> > original 32mb and the new 1 GB ?
>> 
>> But it wasn't "the original 32MB".
>> It was the original "ram_size / 4", until that broke due to argument parsing
>> ordering.
>
> Hmm, 600e17b261555c56a048781b8dd5ba3985650013 says it was 32 MB as the
> default in its commit message, which seems to match the code doing
>
>  #define DEFAULT_CODE_GEN_BUFFER_SIZE_1 (32 * MiB)

You need to look earlier in the sequence (see the tag pull-tcg-20200228):

  47a2def4533a2807e48954abd50b32ecb1aaf29a

so when the argument ordering broke the guest ram_size heuristic we
started getting reports of performance regressions because we fell back
to that size. Before then it was always based on guest ram size within
the min/max bounds set by those defines.

>> I don't know what CI usually uses, but I usually use at least -m 4G, 
>> sometimes
>> more.  What's the libvirt default?
>
> There's no default memory size - its up to whomever/whatever creates the
> VMs to choose how much RAM is given.
>
> Regards,
> Daniel


-- 
Alex Bennée