Re: [PATCH v2] tools/libxl: make default of max event channels dependant on vcpus

[Julien Grall <julien@xxxxxxx>]

Hi Jurgen,

On 06/04/2020 09:27, Juergen Gross wrote:
> Since Xen 4.4 the maximum number of event channels for a guest is
> defaulting to 1023. For large guests with lots of vcpus this is not
> enough, as e.g. the Linux kernel uses 7 event channels per vcpu,
> limiting the guest to about 140 vcpus.
Large guests on which arch? Which type of guests?

> Instead of requiring to specify the allowed number of event channels
> via the "event_channels" domain config option, make the default
> depend on the maximum number of vcpus of the guest. This will require
> to use the "event_channels" domain config option in fewer cases as
> before.
>
> As different guests will have differing needs the calculation of the
> maximum number of event channels can be a rough estimate only,
> currently based on the Linux kernel requirements.
I am not overly happy to extend the default numbers of event channels 
for everyone based on Linux behavior on a given setup. Yes you have more 
guests that would be able to run, but at the expense of allowing a guest 
to use more xen memory.
For instance, I don't think this limit increase is necessary on Arm.

> Nevertheless it is
> much better than the static upper limit of today as more guests will
> boot just fine with the new approach.
>
> In order not to regress current configs use 1023 as the minimum
> default setting.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> V2:
> - use max() instead of min()
> - clarify commit message a little bit
> ---
>   tools/libxl/libxl_create.c | 2 +-
The documentation should be updated.

>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
> index e7cb2dbc2b..c025b21894 100644
> --- a/tools/libxl/libxl_create.c
> +++ b/tools/libxl/libxl_create.c
> @@ -226,7 +226,7 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
>               b_info->iomem[i].gfn = b_info->iomem[i].start;
>   
>       if (!b_info->event_channels)
> -        b_info->event_channels = 1023;
> +        b_info->event_channels = max(1023, b_info->max_vcpus * 8 + 255);
What is the 255 for?

Cheers,

--
Julien Grall