[rrd-developers] Use of madvise / msync kills performance for me

Marcus Reid marcus at blazingdot.com
Thu Jun 26 23:28:11 CEST 2008

On Thu, Jun 26, 2008 at 07:30:03AM +0200, Tobias Oetiker wrote:
> Hi Marcus,
> Have you tried compiling rrrdtool without mmapping ? Note that
> removing msync is BAD. Have a look at the manual page.
>        msync()  flushes  changes  made  to  the in-core copy of a
>        file that was mapped into memory using mmap(2) back to disk.
>        Without use of this call there is no guarantee that changes
>        are written back before munmap(2) is called.

I think things may be different in FreeBSD land.  From the msync
man page:

     The msync() system call is obsolete since BSD implements a coherent file
     system buffer cache.  However, it may be used to associate dirty VM pages
     with file system buffers and thus cause them to be flushed to physical
     media sooner rather than later.

> Obviously it will be faster without this call, but then again,
> the price (potential file corruption) might be a bit high.
> Why some of the madvise calls are are taking so long is unclear to
> me. You might want to try only to drop the WILLNEED calls only and not
> the RANDOM since it is crucial in preserving cache memory ...

Yeah, that part seems odd to me as well.  I'll try asking about that on the
right mailing list and find out what some kernel guys think about it.

> My guess is that the WILLNEED calls actually cause data to be read
> which saves you time later on, your trace does not give absolute
> time, it would be interesting to see that too.

I'll let you know if I find out anything interesting.



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