On Tue, Jan 17, 2023 at 03:54:34PM +0100, Christoph Lameter wrote: > On Tue, 17 Jan 2023, Jesper Dangaard Brouer wrote: > > > When running different network performance microbenchmarks, I started > > to notice that performance was reduced (slightly) when machines had > > longer uptimes. I believe the cause was 'skbuff_head_cache' got > > aliased/merged into the general slub for 256 bytes sized objects (with > > my kernel config, without CONFIG_HARDENED_USERCOPY). > > Well that is a common effect that we see in multiple subsystems. This is > due to general memory fragmentation. Depending on the prior load the > performance could actually be better after some runtime if the caches are > populated avoiding the page allocator etc. > > The merging could actually be beneficial since there may be more partial > slabs to allocate from and thus avoiding expensive calls to the page > allocator. > > I wish we had some effective way of memory defragmentation. If general memory fragmentation is actual cause of this problem, it may be worsening by [1] due to assumption that all allocations are done in the same order as s->oo, when accounting and limiting the number of percpu slabs. [1] https://lore.kernel.org/linux-mm/76c63237-c489-b942-bdd9-5720042f52a9@xxxxxxx
