Re: crm memory usage

Ferenc,

[email protected] wrote:

> crm keeps failing with 'can't fork your minion' messages. (On syscalls.) I
> looked at the code and found that syscall uses fork(), which makes a
> complete copy of the parent process. I'm not an experienced c-programmer
> but I assume the memory usage doubles there and then.

Basically, no. Linux, and I guess most modern systems, use "Copy on Write" (CoW)
strategy. New control blocks are allocated and filled in for the new process,
but the parent and the child continue to use the same pages of memory until one
of them modifies a page. At this moment the page is copied and the two processes
possess of two distinct copies of this particular page. So, only the memory that
is modified is duplicated.

> Is there anything to be done about this. I've read that crm uses memory
> mapping instead of file IO to get performance. I'll happily trade
> performance for less memory usage.

Memory mapping approach is in fact more graceful than "read into memory". Parts
of the file are only being transferred to RAM as the program tries to read that
address in the mapped memory. That's one thing. Another is that multiple
processes that have mmaped the same file (read-only) all use the same RAM pages.

Eugene

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