On Wed, Jun 17, 2020 at 01:24:21PM +0200, Vlastimil Babka wrote: > > Not really. > > > > Sharing a single set of caches adds some overhead to root- and non-accounted > > allocations, which is something I've tried hard to avoid in my original version. > > But I have to admit, it allows to simplify and remove a lot of code, and here > > it's hard to argue with Johanness, who pushed on this design. > > > > With performance testing it's not that easy, because it's not obvious what > > we wanna test. Obviously, per-object accounting is more expensive, and > > measuring something like 1000000 allocations and deallocations in a line from > > a single kmem_cache will show a regression. But in the real world the relative > > cost of allocations is usually low, and we can get some benefits from a smaller > > working set and from having shared kmem_cache objects cache hot. > > Not speaking about some extra memory and the fragmentation reduction. > > > > We've done an extensive testing of the original version in Facebook production, > > and we haven't noticed any regressions so far. But I have to admit, we were > > using an original version with two sets of kmem_caches. > > > > If you have any specific tests in mind, I can definitely run them. Or if you > > can help with the performance evaluation, I'll appreciate it a lot. > > Jesper provided some pointers here [1], it would be really great if you could > run at least those microbenchmarks. With mmtests it's the major question of > which subset/profiles to run, maybe the referenced commits provide some hints, > or maybe Mel could suggest what he used to evaluate SLAB vs SLUB not so long ago. > Last time the list of mmtests configurations I used for a basic comparison were db-pgbench-timed-ro-small-ext4 db-pgbench-timed-ro-small-xfs io-dbench4-async-ext4 io-dbench4-async-xfs io-bonnie-dir-async-ext4 io-bonnie-dir-async-xfs io-bonnie-file-async-ext4 io-bonnie-file-async-xfs io-fsmark-xfsrepair-xfs io-metadata-xfs network-netperf-unbound network-netperf-cross-node network-netperf-cross-socket network-sockperf-unbound network-netperf-unix-unbound network-netpipe network-tbench pagereclaim-shrinker-ext4 scheduler-unbound scheduler-forkintensive workload-kerndevel-xfs workload-thpscale-madvhugepage-xfs workload-thpscale-xfs Some were more valid than others in terms of doing an evaluation. I followed up later with a more comprehensive comparison but that was overkill. Each time I did a slab/slub comparison in the past, I had to reverify the rate that kmem_cache_* functions were actually being called as the pattern can change over time even for the same workload. A comparison gets more complicated when comparing cgroups as ideally there would be workloads running in multiple group but that gets complex and I think it's reasonable to just test the "basic" case without cgroups. -- Mel Gorman SUSE Labs
