It means they are also limited to 3 gigs maximum. The split is not 1-gig for the kernel and 3-gigs for processes, it is 1-gig for the kernel and 3-gigs PER process.

The problem is that a 32-bit address space only gives you 4 gigs of addressable memory, so the processor has an "interpreter" called the Translation Lookaside Buffer which maps virtual addresses into physical addresses. So your perl application sees the system as having 4 gigs of memory, 1 of which is for the kernel and 3 gigs for itself. Other processes see the same thing, but the 3 gigs they are able to address may not be the same 3 gigs of physical memory that other processes see. The upside to this arrangment is that it lets you use more than 4 gigs of RAM in a 32-bit machine. The downside is that the TLB must be flushed when a different process is swapped onto the processor because the addresses that were valid for one process may not be the same addresses for another process.

If memory is the only problem, besides the option to go to 64-bit hardware you could also look at getting (or building) a 4-gig/4-gig split kernel. Red Hat calls this kernel the 'hugemem' kernel, other distros probably have something similar. Using this kernel the kernel gets 4 gigs of address space and processes get 4 gigs each. The downside to this is that the TLB can still only deal with 4 gigs of address space, so with the 3/1 split kernel you have to flush the TLB every time you switch processes, with the 4/4 split kernel you also have to flush the TLB every time you switch from kernel mode to user mode or vice-versa. How much of an impact this has on performance varies (mostly depending on how much I/O your application does), but taking a 60% performance hit for using a 4/4 split kernel is not uncommon.

We're not surrounded, we're in a target-rich environment!

In reply to Re^2: How can I let Perl use more than 3GB of RAM?? by jasonk
in thread How can I let Perl use more than 3GB of RAM?? by maya_the_be

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