Understanding the locking behavior of msync

[Mulyadi Santosa]

On Fri, Mar 26, 2021, 22:05 Maximilian Böther <
maximilian.boether at 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-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
>
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Is it NUMA?
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