Recently on this list the idea of 'pinning' or locking the root apache process in memory has been discussed with some interest. The reason being was that some users have experienced the situtaion where a server becomes loaded, and the root apache process gets swapped out, and in the process loses some of its shared memory. Future child processes that are forked also share in the loss of shared memory, so methods like using GtopLimit to 'recyle' child processes when their shared memory becomes too low cease to work because when they come up, they are already too low on shared memory.
In our systems we had attempted this but it always came down to the same problem --the root process would lose its shared memory, to the point that any child process would come up, serve a request, find that it was beyond the threshold for shared memory and die. The only help was to restart Apache altogether. So in scouring the list I found someone mentioning using the mlockall C function to lock the pages of the core apache process in memory. Some handy .xs code was provided, so I built a module, Sys::Mman, which wraps mlockall, and makes it available to Perl. We installed this on our servers, and call mlockall right at the end of our preload stuff, i.e., the end of the 'startup.pl'-style script called from httpd.conf. The result has been very encouraging. The core apache process is able then to maintain all its shared memory, and child processes that are forked are able to start with high amounts of shared memory, all making for a much happier system. Now I also read that probably better than this would be to ensure that you never swap by tuning MaxClients, as well as examining our Perl code to make it less prone to lose shared memory. We're working on that sort of tuning, but in volatile environments like ours, where we serve a very large amount of data, and new code is coming out almost daily here and there, locking the core httpd in memory has been very helpful. I just thought I would let others know on the list that it is feasible, and works well in our environment. If there's enough interest I might put the module up on CPAN, but it's really very simple. h2xs did most of the work for me. And thanks to Doug MacEachern for posting the .xs code. It worked like a charm. FWIW, -- Dan Hanks ======================================================================== Daniel Hanks - Systems/Database Administrator About Inc., Web Services Division ========================================================================
