Re: [PATCH v1 5/5] accel/tcg: better handle memory constrained systems

[Daniel P . Berrangé]

On Fri, Jul 17, 2020 at 11:51:39AM +0100, Alex Bennée wrote:
> It turns out there are some 64 bit systems that have relatively low
> amounts of physical memory available to them (typically CI system).
> Even with swapping available a 1GB translation buffer that fills up
> can put the machine under increased memory pressure. Detect these low
> memory situations and reduce tb_size appropriately.
> 
> Fixes: 600e17b261
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: BALATON Zoltan <balaton@eik.bme.hu>
> Cc: Christian Ehrhardt <christian.ehrhardt@canonical.com>
> ---
>  accel/tcg/translate-all.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index 2afa46bd2b1..2ff0ba6d19b 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -976,7 +976,12 @@ static inline size_t size_code_gen_buffer(size_t tb_size)
>  {
>      /* Size the buffer.  */
>      if (tb_size == 0) {
> -        tb_size = DEFAULT_CODE_GEN_BUFFER_SIZE;
> +        size_t phys_mem = qemu_get_host_physmem();
> +        if (phys_mem > 0 && phys_mem < (2 * DEFAULT_CODE_GEN_BUFFER_SIZE)) {
> +            tb_size = phys_mem / 4;
> +        } else {
> +            tb_size = DEFAULT_CODE_GEN_BUFFER_SIZE;
> +        }

I'm not convinced this is going to work when running QEMU inside a
container environment with RAM cap, because the physmem level is
completely unrelated to the RAM the container is permitted to actually
use in practice. ie host has 32 GB of RAM, but the container QEMU is
in only has 1 GB permitted.

I don't have much of a better suggestion, as I don't think we want
to get into reading the cgroups memory limits. It does feel like the
assumption we can blindly use a 1GB cache though is invalid even
with this patch applied.

Regards,
Daniel
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