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Out of curiosity — was the “bad” mode a case of two separate workloads competing for limited cache within one CCX, or was it really bad cache behaviour across C
by pdpi 5y ago
Out of curiosity — was the “bad” mode a case of two separate workloads competing for limited cache within one CCX, or was it really bad cache behaviour across CCXs causing problems because the two processes shared memory?
- kentonv 5y agoThe two processes were a client and a server, the client just sends HTTP requests to the server over a unix socket, but otherwise they don't share resources. TBH I'm surprised there was so much impact just from that. There might be something more to the story -- I haven't investigated much yet. But basically when I used `taskset` to limit the benchmark to two cores in different CCX's, it ran about half the speed as when limited to two cores in the same CCX. I guess another possibility is that the kernel is allowing both processes to swap between cores regularly, which would much better explain the disastrous performance penalty. But, in that case I don't understand the bimodal distribution I see when running the benchmark with no core affinity...