On 7/23/23 21:09, Hyeonggon Yoo wrote: > By default, SLUB sets remote_node_defrag_ratio to 1000, which makes it > (in most cases) take slabs from remote nodes first before trying allocating > new folios on the local node from buddy. > > Documentation/ABI/testing/sysfs-kernel-slab says: >> The file remote_node_defrag_ratio specifies the percentage of >> times SLUB will attempt to refill the cpu slab with a partial >> slab from a remote node as opposed to allocating a new slab on >> the local node. This reduces the amount of wasted memory over >> the entire system but can be expensive. > > Although this made sense when it was introduced, the portion of > per node partial lists in the overall SLUB memory usage has been decreased > since the introduction of per cpu partial lists. Therefore, it's worth > reevaluating its overhead on performance and memory usage. > > [ > XXX: Add performance data. I tried to measure its impact on > hackbench with a 2 socket NUMA machine. but it seems hackbench is > too synthetic to benefit from this, because the skbuff_head_cache's > size fits into the last level cache. > > Probably more realistic workloads like netperf would benefit > from this? > ] > > Set remote_node_defrag_ratio to zero by default, and the new behavior is: > 1) try refilling per CPU partial list from the local node > 2) try allocating new slabs from the local node without reclamation > 3) try refilling per CPU partial list from remote nodes > 4) try allocating new slabs from the local node or remote nodes > > If user specified remote_node_defrag_ratio, it probabilistically tries > 3) first and then try 2) and 4) in order, to avoid unexpected behavioral > change from user's perspective. It makes sense to me, but as you note it would be great to demonstrate benefits, because it adds complexity, especially in the already complex ___slab_alloc(). Networking has been indeed historically a workload very sensitive to slab performance, so seems a good candidate. We could also postpone this until we have tried the percpu arrays improvements discussed at LSF/MM.
