Apr 6, 2013 · On at least one system, man msync says: The msync() system call writes modified whole pages back to the filesystem and updates the file modification time. Only those pages containing addr and len-1 succeeding locations will be examined.

msync is not supposed to have anything to do with whether the changes are committed to the logical state of the file, or whether other processes using mmap or read to access the file see the changes; it's purely an analogue of fsync and should be treated as a no-op except for the purposes of ensuring data integrity in the event of power failure ...

Actually, msync is largely a no-op on Linux or any system with a proper virtual memory and page cache system; read will immediately see anything written to the mmapped pages, even without msync. It's largely an analog of fsync , but with a memory address range rather than a file descriptor as its argument.

Feb 12, 2020 · Linux version: 4.19 Platform: Xilinx Ultrascale+ Zynq In the programmable logic I've created a memory mapped device located at physical address 0xA0001000. I'm using uio_pdrv_genirq as my device

Oct 19, 2017 · On Linux msync(MS_ASYNC) is a no-op (see the VERSIONS section of the msync() man page). On other OSes it may do something and of course you can call msync() with other parameters. If you need to know whether syncing completed and you're willing to block until it has, you can use msync(MS_SYNC) (notice the lack the A).

Mar 6, 2009 · When I read the man page for msync, I would not assume that it is actually flushing the disk cache (the cache in the disk unit, as opposed to the system cache in main memory). FlushViewOfFile won't return until the disk stack has completed the writes; like the msync documentation, it says nothing about what happens in the disk cache.

Aug 18, 2021 · In the current Linux kernel implementation, is there a performance penalty for using msync on the whole file? For example if the file is 100GB, but I only made total of 10MB changes? Is the kernel looping over every page in the range provided for msync to find the dirty pages to flush or are those kept on some sort of linked list/other structure?

I'm currently using OpenLDAP Symas MDB as a persistent key/value storage and without MDB_MAPASYNC - which results in using msync(MS_ASYNC) (I looked through the source code) - the writes are so slow, that even while processing data a single core is permanently waiting on IO at sometimes < 1MB/s. After analyzing, the problem seems to be many ...

Jun 9, 2011 · The msync man page states: ENOMEM The indicated memory (or part of it) was not mapped. That's the errno value perror() prints for you. So you're somehow trying to msync() memory that you've not mmap()'ed from a file.

Sep 29, 2014 · Perhaps more to the point, man 2 msync has this note: Since Linux 2.6.19, MS_ASYNC is in fact a no-op, since the kernel properly tracks dirty pages and flushes them to storage as necessary. Remember: mmap is directly exposing pages of the filesystem cache to userspace and, just like any other caching system, changes will be propagated to the ...