Hi Scott, > On Wed, 20 Mar 2002, Stas Bekman wrote: > >>mod_perl child processes save a lot of memory when they can share memory >>with the parent process and quite often we get reports from people that >>they lose that shared memory when the system decides to page out the >>parent's memory pages because they are LRU (least recently used, the >>algorithm used by many memory managers). >> > > I'm fairly certain that this is not an issue. If a page was shared COW > before being paged out, I expect it will be shared COW when paged back in, > at least for any modern OS.
But if the system needs to page things out, most of the parent process's pages will be scheduled to go first, no? So we are talking about a constant page-in/page-out from/to the parent process as a performance degradation rather than memory unsharing. Am I correct? > [To verify that I wasn't talking through my hat, here, I just verified > this using RedHat 7.2 running kernel 2.4.9-21. If you're interested in my > methodology, drop me an email.] I suppose that this could vary from one kernel version to another. I'm just repeating the reports posted to the mod_perl list. I've never seen such a problem myself, since I try hard to have close to zero swap usage. [Yes, please let me know your methodology for testing this] >>I believe that this applies to all httpd modules and httpd itself, the >>more we can share the less memory resources are needed, and usually it >>leads to a better performance. >> > > I'm absolutely _certain_ that unmodified pages from executable files will > be backed by the executable, and will thus be shared by default. > > >>Therefore my question is there any reason for not using mlockall(2) in >>the parent process on systems that support it and when the parent httpd >>is started as root (mlock* works only within root owned processes). >> > > I don't think mlockall is appropriate for something with the heft of > mod_perl. > > Why are the pages being swapped out in the first place? Presumably > there's a valid reason. Well, the system coming close to zero of real memory available. The parent process starts swapping like crazy because most of its pages are LRU, slowing the whole system down and if the load doesn't go away, the system takes a whirl down to a halt. > Doing mlockall on your mod_perl would result in > restricting the memory available to the rest of the system. Whatever is > causing mod_perl to page out would then start thrashing. Worse, since > mlockall will lock down mod_perl pages indiscriminately, the resulting > thrashing will probably be even worse than what they're seeing right now. Possible, I've never tried this myself and therefore asked. Someone has suggested using P_SYSTEM flag which is supposed to tell the system not to page out certain pages, but seems to be a *BSD thingy. I've also seen madvise(2) - "give advice about use of memory" but again no personal experience here. _____________________________________________________________________ Stas Bekman JAm_pH -- Just Another mod_perl Hacker http://stason.org/ mod_perl Guide http://perl.apache.org/guide mailto:[EMAIL PROTECTED] http://ticketmaster.com http://apacheweek.com http://singlesheaven.com http://perl.apache.org http://perlmonth.com/
