On Thu, 18 May 2000, Erik Corry wrote:
> If I mmap 1 Gbyte of VM (anonymously, privately from /dev/zero) but
> don't use much of it, how much RAM does the system spend managing
> it? Is there a fixed per-page (4k) overhead for the PTEs?
One page (4 kB) per 4 MB of memory, so 1 MB overhead for
your 1 GB mapping.
> Also, why does top misreport the RSS as 1Gbyte in this case
> (kernel 2.2.5-22, procps-2.0.2-2)? Kernel bug?
The pages you have not written to all point to the
zero-filled page. If you set up the mapping you'll
have 256k mappings to that one page :)
> Do I need more real RAM to fork if I have a large mmap like this?
No. You will only need more RAM (or swap) if you
write to a lot of the pages, otherwise they'll
keep pointing (COW) to the zero-filled page.
regards,
Rik
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