Messages in this thread | | | Date | Wed, 4 Apr 2007 17:27:31 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [rfc] no ZERO_PAGE? |
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On Wed, 4 Apr 2007, Valdis.Kletnieks@vt.edu wrote: > > I'd not be surprised if there's sparse-matrix code out there that wants to > malloc a *huge* array (like a 1025x1025 array of numbers) that then only > actually *writes* to several hundred locations, and relies on the fact that > all the untouched pages read back all-zeros.
Good point. In fact, it doesn't need to be a malloc() - I remember people doing this with Fortran programs and just having an absolutely incredibly big BSS (with traditional Fortran, dymic memory allocations are just not done).
> Of course, said code is probably buggy because it doesn't zero the whole > thing because you don't usually know if some other function already > scribbled on that heap page.
Sure you do. If glibc used mmap() or brk(), it *knows* the new data is zero. So if you use calloc(), for example, it's entirely possible that a good libc wouldn't waste time zeroing it.
The same is true of BSS. You never clear the BSS with a memset, you just know it starts out zeroed.
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