Re: Better pagecache statistics ?

[Tom Zanussi <zanussi at us dot ibm dot com>]

Marcelo Tosatti writes:

[...]

 > 
 > b) ERROR: MAXACTION exceeded near identifier 'log' at ttfp_delay.stp:49:3
 > 
 > The array size is capped to a maximum. Is there any way to configure
 > SystemTap to periodically dump-and-zero the arrays? This makes lots of
 > sense to any statistical gathering code.
 > 
 > c) Hash tables
 > 
 > It would be better to store the log entries in a hash table, the present
 > script uses the "current" pointer as a key into a pair of arrays,
 > incrementing the key until a free one is found (which can be very
 > inefficient).
 > 
 > A hash table would be much more efficient, but allocating memory inside
 > the scripts is tricky. A pre-allocated, pre-sized pool of memory could 
 > work well for this purpose. The "dump-array-entries-to-userspace" action
 > could be used to free them.
 > 
 > So both b) and c) could be fixed with the same logic:
 > 
 > - dump entries to userspace if memory pool is getting short 
 > on free entries.
 > - periodically dump entries to userspace (akin to "bdflush").

Hi,

There's a sytemtap example that does something similar to what you're
describing - see the kmalloc-stacks/kmalloc-top examples in the
testsuite:

systemtap/tests/systemtap.samples/kmalloc-stacks.stp
systemtap/tests/systemtap.samples/kmalloc-top

Basically, the kmalloc-stacks.stp script hashes data in a systemtap
hash and periodically formats the current contents of the hash table
into a convenient form and writes it to userspace, then clears the
hash for the next go-round.  kmalloc-top is a companion Perl script
'daemon' that sits around in userspace waiting for new batches of hash
data, which it then adds to a continuously accumulating Perl hash in
the user-side script.  There's a bit more detail about the script(s)
here:

http://sourceware.org/ml/systemtap/2005-q3/msg00550.html

HTH,

Tom