msync() differences between Linux and FreeBSD

[Marcus Reid]

Hi,

I recently had a patch accepted to the ports collection that took out an msync()
call that seriously detrimented performance for rrdtool updates.  It seems that
in FreeBSD, msync() waits for bits to be committed to disk even when MS_ASYNC
is specified.  Under Linux, there is not such a wait for msync().

First off, I don't know how frequently msync() is used, and whether changing
its behavior would impact the performance of many things.

It's possible that FreeBSD msync() behavior is more correct in some ways. From
a message from Matt Dillon on this same list:

    It used to be that msync() only synced VM pages to the underlying
    file, making them consistent with read()'s and write()'s against
    the underlying file.  Since FreeBSD uses a unified VM page cache
    this is always true.  However, the Open Group specification now
    requires that the dirty pages actually be written out to the underlying
    media... i.e. issue real I/O.  So msync() can't be a NOP if you go by
    the OpenGroup specification.

Is there a spec that FreeBSD is adhering to that prevents msync() with
MS_ASYNC from being a NOP, seeing as munmap() does the job?  And does this
really matter for the real-world performance of some apps?

Thanks,

Marcus