Messages in this thread |  | | Date | Tue, 7 Jan 2020 11:42:11 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains v2 |
| |
On Tue, Jan 07, 2020 at 12:22:55PM +0100, Peter Zijlstra wrote: > > Much more importantly, doing what you suggest allows an imbalance > > of more CPUs than are backed by a single LLC. On high-end AMD EPYC 2 > > machines, busiest->group_weight scaled by imbalance_pct spans multiple L3 > > caches. That is going to have side-effects. While I also do not account > > for the LLC group_weight, it's unlikely the cut-off I used would be > > smaller than an LLC cache on a large machine as the cache. > > > > These two points are why I didn't take the group weight into account. > > > > Now if you want, I can do what you suggest anyway as long as you are happy > > that the child domain weight is also taken into account and to bound the > > largest possible allowed imbalance to deal with the case of a node having > > multiple small LLC caches. That means that some machines will be using the > > size of the node and some machines will use the size of an LLC. It's less > > predictable overall as some machines will be "special" relative to others > > making it harder to reproduce certain problems locally but it would take > > imbalance_pct into account in a way that you're happy with. > > > > Also bear in mind that whether LLC is accounted for or not, the final > > result should be halved similar to the other imbalance calculations to > > avoid over or under load balancing. > > > + /* Consider allowing a small imbalance between NUMA groups */ > > + if (env->sd->flags & SD_NUMA) { > > + struct sched_domain *child = env->sd->child; > > This assumes sd-child exists, which should be true for NUMA domains I > suppose. >
I would be stunned if it was not. What sort of NUMA domain would not have child domains? Does a memory-only NUMA node with no CPUs even generate a scheduler domain? If it does, then I guess the check is necessary.
> > + unsigned int imbalance_adj; > > + > > + /* > > + * Calculate an acceptable degree of imbalance based > > + * on imbalance_adj. However, do not allow a greater > > + * imbalance than the child domains weight to avoid > > + * a case where the allowed imbalance spans multiple > > + * LLCs. > > + */ > > That comment is a wee misleading, @child is not an LLC per se. This > could be the NUMA distance 2 domain, in which case @child is the NUMA > distance 1 group. > > That said, even then it probably makes sense to ensure you don't idle a > whole smaller distance group. >
I hadn't considered that case but even then, it's just a comment fix. Thanks.
-- Mel Gorman SUSE Labs
|  |