Re: [PATCH v2 1/2] credit: Limit load balancing to once per millisecond

[Juergen Gross <jgross@xxxxxxxx>]

On 21.09.23 14:23, George Dunlap wrote:
> The credit scheduler tries as hard as it can to ensure that it always
> runs scheduling units with positive credit (PRI_TS_UNDER) before
> running those with negative credit (PRI_TS_OVER).  If the next
> runnable scheduling unit is of priority OVER, it will always run the
> load balancer, which will scour the system looking for another
> scheduling unit of the UNDER priority.
>
> Unfortunately, as the number of cores on a system has grown, the cost
> of the work-stealing algorithm has dramatically increased; a recent
> trace on a system with 128 cores showed this taking over 50
> microseconds.
>
> Add a parameter, load_balance_ratelimit, to limit the frequency of
> load balance operations on a given pcpu.  Default this to 1
> millisecond.
>
> Invert the load balancing conditional to make it more clear, and line
> up more closely with the comment above it.
>
> Overall it might be cleaner to have the last_load_balance checking
> happen inside csched_load_balance(), but that would require either
> passing both now and spc into the function, or looking them up again;
> both of which seemed to be worse than simply checking and setting the
> values before calling it.
>
> On a system with a vcpu:pcpu ratio of 2:1, running Windows guests
> (which will end up calling YIELD during spinlock contention), this
> patch increased performance significantly.
>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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