Re: [Xen-devel] [libvirt] [PATCH 2/2] libxl: support vif outgoing bandwidth QoS

[Michal Privoznik <mprivozn@xxxxxxxxxx>]

On 29.12.2015 02:09, Jim Fehlig wrote:
> The libxl_device_nic structure supports specifying an outgoing rate
> limit based on a time interval and bytes allowed per interval. In xl
> config a rate limit is specified as "<RATE>/s@<INTERVAL>". INTERVAL
> is optional and defaults to 50ms.
> 
> libvirt expresses outgoing limits by average (required), peak, burst,
> and floor attributes in units of KB/s. This patch supports the outgoing
> bandwidth limit by converting the average KB/s to bytes per interval
> based on the same default interval (50ms) used by xl.
> 
> Signed-off-by: Jim Fehlig <jfehlig@xxxxxxxx>
> ---
>  src/libxl/libxl_conf.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/src/libxl/libxl_conf.c b/src/libxl/libxl_conf.c
> index 23c74e7..6320421 100644
> --- a/src/libxl/libxl_conf.c
> +++ b/src/libxl/libxl_conf.c
> @@ -1093,6 +1093,7 @@ libxlMakeNic(virDomainDefPtr def,
>  {
>      bool ioemu_nic = def->os.type == VIR_DOMAIN_OSTYPE_HVM;
>      virDomainNetType actual_type = virDomainNetGetActualType(l_nic);
> +    virNetDevBandwidthPtr actual_bw;
>  
>      /* TODO: Where is mtu stored?
>       *
> @@ -1206,6 +1207,44 @@ libxlMakeNic(virDomainDefPtr def,
>  #endif
>      }
>  
> +    /*
> +     * Set bandwidth.
> +     * From $xen-sources/docs/misc/xl-network-configuration.markdown:
> +     *
> +     *
> +     * Specifies the rate at which the outgoing traffic will be limited to.
> +     * The default if this keyword is not specified is unlimited.
> +     *
> +     * The rate may be specified as "<RATE>/s" or optionally 
> "<RATE>/s@<INTERVAL>".
> +     *
> +     * `RATE` is in bytes and can accept suffixes:
> +     *     GB, MB, KB, B for bytes.
> +     *     Gb, Mb, Kb, b for bits.
> +     * `INTERVAL` is in microseconds and can accept suffixes: ms, us, s.
> +     *     It determines the frequency at which the vif transmission credit
> +     *     is replenished. The default is 50ms.
> +
> +     * Vif rate limiting is credit-based. It means that for "1MB/s@20ms",
> +     * the available credit will be equivalent of the traffic you would have
> +     * done at "1MB/s" during 20ms. This will results in a credit of 20,000
> +     * bytes replenished every 20,000 us.
> +     *
> +     *
> +     * libvirt doesn't support the notion of rate limiting over an interval.
> +     * Similar to xl's behavior when interval is not specified, set a default
> +     * interval of 50ms and calculate the number of bytes per interval based
> +     * on the specified average bandwidth.
> +     */
> +    actual_bw = virDomainNetGetActualBandwidth(l_nic);
> +    if (actual_bw && actual_bw->out && actual_bw->out->average) {
> +        uint64_t bytes_per_sec = actual_bw->out->average * 1024;
> +        uint64_t bytes_per_interval =
> +            (((uint64_t) bytes_per_sec * 50000UL) / 1000000UL);
> +
> +        x_nic->rate_bytes_per_interval = bytes_per_interval;
> +        x_nic->rate_interval_usecs =  50000UL;
> +    }
> +

Interesting. I'd expect:

x_nic->rate_bytes_per_interval = bytes_per_sec;
x_nic->rate_interval_usecs = 1000*1000;

I mean, if I understood the xl way of rate limiting correctly, one says
how much bytes can be sent for how long. so for 1MB/s I'd expect to send
1024*1024 bytes each second.

Or am I missing something?

Michal

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