Sunday, March 14, 2010

Improving the access logging code

The Squid/Lusca access logging logic is .. well, to be honest, it's quite dirty and inefficient. Yes, like most of Squid. But this is a somewhat bite-sized piece of the puzzle which I can tidy up in one corner without too many effects elsewhere.. or is it?

For high request rate, small object loads on slower FSB hardware, one of the largest CPU users is actually the client-side access log path. There's two culprits here:
  1. memcpy() when copying the "hier" struct from the request struct to the access log entry struct (client_side.c:297; revision 14457); and
  2. The ACL list setup in preparation for writing filtered access log entries to multiple files (client_side.c:314; revision 14457).
The memcpy() is of a 588 byte HierarchyLogEntry struct - it's this large because of two "SQUIDHOSTNAMELEN" (256 byte) long strings embedded in the struct itself. Annoying, but somewhat fixable with a little code refactoring and use of reference counted strings.

The ACL list setup is a bit more problematic. It sets up an ACL checklist using the http_access checklist before then checking log_access / acl_access. It then further may use this ACL checklist when evaluating various access_log lines to allow filtering certain access log entries to certain files.

Now, the latter bites because the really slow paths (setting up and destroying the ACL access stuff) is done even if the default logging configuration is used - one log file - and there's currently no easy way around that. Furthermore, if you disable logging entirely (access_log none) then the initial setup of the access log entry information is done, the ACL checklist is created, and then it's all tossed away without logging. A bit stupid, eh?

I'll cover what I'm doing in the codebase in a subsequent post (read; when I'm not running off to class.)

No comments:

Post a Comment

Post a Comment