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

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Richard Henderson <richard.henderson@linaro.org>
Cc: fam@euphon.net, "Alex Bennée" <alex.bennee@linaro.org>,
	qemu-devel@nongnu.org,
	"Christian Ehrhardt" <christian.ehrhardt@canonical.com>,
	f4bug@amsat.org, cota@braap.org,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	aurelien@aurel32.net, "Richard Henderson" <rth@twiddle.net>
Subject: Re: [PATCH v2 06/12] accel/tcg: better handle memory constrained systems
Date: Thu, 23 Jul 2020 10:00:27 +0100	[thread overview]
Message-ID: <20200723090027.GB2615312@redhat.com> (raw)
In-Reply-To: <0efbcac2-d2f4-de96-63a1-ba7cd485a1e6@linaro.org>

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)

> 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
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