[mythtv-users] pdflush is killing me

[Dan Ritter]

On Wed, Feb 07, 2007 at 08:39:59AM -0500, Steven Adeff wrote:
> having seen pdflush run while observing top (not knowing if it was
> causing any actual issues...), I've taken an interest in what you've
> written here. Would you mind elaborating on what this does for people
> like me that don't know what that means?

Linux uses any spare RAM you might have for caching disk reads
and writes. RAM is, of course, much faster than disk, and
writing sequentially to disk is faster than writing to random
locations. So, the longer you can go between an application
trying to write some data and actually doing that write to disk,
the better your performance will be.

Unfortunately, data that's in the cache but not written to disk
will disappear in a power glitch. Also, applications are always
asking for more memory, and the cached data has a lower priority
claim. pdflush is a kernel-controlled process that takes that
dirty (i.e. written to memory but not yet on disk, as opposed to
clean, which is the same on disk and in memory) information and
sends it all to disk. It can be tuned to do so more or less
often, but the usual default is 5 seconds.

The kernel parameter dirty_writeback_centisecs controls the time
between pdflush flushes. dirty_expire_centisecs controls the
minimum age of a dirty page before pdflush is allowed to flush
it. (That guarantees that some caching happens.) 

-dsr-


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