On Fri, Mar 26, 2021, 22:05 Maximilian Böther < maximilian.boether@student.hpi.de> wrote:
Hello!
I am investigating an application that writes random data in fixed-size chunks (e.g. 4k) to random locations in a large buffer file. I have several processes (not threads) doing that, each process has its own buffer file assigned.
If I use mmap+msync to write and persist data to disk, I see a performance spike for 16 processes, and a performance drop for more threads (32 processes). The CPU has 32 logical cores in total, and we are not CPU bound.
If I use open+write+fsync, I do not see such a spike, instead a performance plateau (and mmap is slower than open/write).
I've read multiple times [1,2] that both mmap and msync can take locks. With vtune, I analyzed that we are indeed spinlocking, and spending the most time in clear_page_erms and xas_load functions.
However, when reading the source code for msync [3], I cannot understand whether these locks are global or per-file. The paper [2] states that the locks are on radix-trees within the kernel that are per-file, however, as I do observe some spinlocks in the kernel, I believe that some locks may be global, as I have one file per process.
Do you have an explanation on why we have such a spike at 16 processes for mmap and input on the locking behavior of msync?
Thank you!
Best, Maximilian Böther
[1]
https://kb.pmem.io/development/100000025-Why-msync-is-less-optimal-for-persi... - I know it's about PMem, but the lock argument is important
[2] Optimizing Memory-mapped I/O for Fast Storage Devices, Papagiannis et al., ATC '20
[3] https://elixir.bootlin.com/linux/latest/source/mm/msync.c
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Is it NUMA?