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

[Richard Henderson]

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.

> 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.

I don't know what CI usually uses, but I usually use at least -m 4G, sometimes
more.  What's the libvirt default?


r~