What's your problem then ?

What's your problem then ?

Posted Oct 11, 2007 23:25 UTC (Thu) by jzbiciak (✭ supporter ✭, #5246)
In reply to: What's your problem then ? by khim
Parent article: Memory part 3: Virtual Memory

There's a pretty big difference between the "last page" of "an allocation several pages long", and "each mapping". Each page has a mapping, and so the allocation mentioned has multiple mappings associated with it because it involves multiple pages. Suppose I allocate 62K, and I need to carve it into 4K pages. I end up allocating 64K total (16 pages). 15 of the 16 pages are fully utilized, and the last page is half-wasted.

Consider an allocation of N bytes that does not share pages with any other allocation--for example, an allocation via mmap.

• It requires ceil(N / PAGE_SIZE) pages (and therefore mappings).
• It will waste (PAGE_SIZE - (N % PAGE_SIZE)) bytes total.
• There will be floor(N / PAGE_SIZE) fully utilized pages.

The amount of wasted bytes depends on the distribution of N. As N grows larger (and the number of mappings per allocation goes up), the number of wasted bytes per mapping goes down.

(And yes, that's pseudo-code. The actual C code to compute those quantities isn't much different, but it's a little noisier.)

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