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hosay123 428 days ago | link | parent

    ptr = mmap(..., len, ..)
    /* do stuff with ptr */
    msync(ptr, len)
No sane OS will pay attention to an mmapped region while it isn't under memory pressure, so dirty pages are effectively buffered until you explicitly tell the OS to start writeback.


otterley 428 days ago | link

Linux flushes dirty pages in mmapped regions to disk every 30 seconds by default whether you like it or not (see vm.dirty_expire_centisecs). Unlike FreeBSD, Linux doesn't yet have support for the MAP_NOSYNC flag to mmap(2).

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hosay123 428 days ago | link

Good point, though I think for a machine averaging even 40mb/sec this only amounts to one 'batch' every 1.2gb receiving an extra sync. Linux recently changed so that pdflush doesn't exist at all, when the dirty list fills, a per-device thread is created that sleeps for 5 seconds before starting work, so maybe it's a bit less than 1.2gb.

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riobard 428 days ago | link

Does this mean that I have to msync multiple mmap'ed chunks in order to batch write? For example

    msync(ptr1, len1)
    msync(ptr2, len2)
    msync(ptr3, len3)
Where [ptr1, ptr1+len1], [ptr2, ptr2+len2], ... are the chunks within a big mmap'ed region where changes occur and need to be written to disk for persistence.

Or do I just msync the whole region then hope and pray that the OS will do the right thing?

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