Hi I performed some microbenchmarks on a system with real Optane-based nvdimm and I found out that the fastest method how to write to persistent memory is to fill a cacheline with 8 8-byte writes and then issue clwb or clflushopt on the cacheline. With this method, we can achieve 1.6 GB/s throughput for linear writes. On the other hand, non-temporal writes achieve only 1.3 GB/s. The results are here: http://people.redhat.com/~mpatocka/testcases/pmem/microbenchmarks/pmem.txt The benchmarks here: http://people.redhat.com/~mpatocka/testcases/pmem/microbenchmarks/ The winning benchmark is this: http://people.redhat.com/~mpatocka/testcases/pmem/microbenchmarks/thrp-write-8-clwb.c However, the kernel is not using this fastest method, it is using non-temporal stores instead. I took the novafs filesystem (see git clone https://github.com/NVSL/linux-nova), it uses __copy_from_user_inatomic_nocache, which calls __copy_user_nocache which performs non-temporal stores. I hacked __copy_user_nocache to use clwb instead of non-temporal stores and it improved filesystem performance significantly. This is the patch http://people.redhat.com/~mpatocka/testcases/pmem/benchmarks/copy-nocache.patch (for the kernel 5.1 because novafs needs this version) and these are benchmark results: http://people.redhat.com/~mpatocka/testcases/pmem/benchmarks/fs-bench.txt - you can see that "test2" has twice the write throughput of "test1" I took the dm-writecache driver, it uses memcpy_flushcache to write data to persistent memory. I hacked memcpy_flushcache to use clwb instead of non-temporal stores. The result is - for 512-byte writes, non-temporal stores perform better than cache flushing. For 1024-byte and larger writes, cache flushing performs better than non-temporal stores. (I also tried to use cached writes + clwb for dm-writecache metadata updates, but it had bad performance) Do you have some explanation why nontemporal stores are better for 512-byte copies and worse for 1024-byte copies? (like filling up some buffers inside the CPU)? In the next email, I'm sending a patch that makes memcpy_flushcache use clflushopt for transfers larger than 768 bytes. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
