Understanding the locking behavior of msync

[Maximilian Böther]

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-persistent-memory/ 
- 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