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Re: Whats the real penalty of non-mmap ggc?
- From: Geoffrey Keating <geoffk at geoffk dot org>
- To: jkj at sco dot com, gcc at gcc dot gnu dot org
- Date: 12 Oct 2005 10:46:46 -0700
- Subject: Re: Whats the real penalty of non-mmap ggc?
- References: <434D43EA.1090804@sco.com>
Kean Johnston <jkj@sco.com> writes:
> Hi all,
>
> After days spent trying to get a clean gmake check run, I am down to
> the last few failures. They are all related to the PCH tests, and they
> all
> fail the same way: "largefile.c:1: fatal error: had to relocate PCH".
> Previously, *all* PCH tests were failing but I crafted a host file
> that uses MAP_FIXED and now only this one test fails. However, if I
> pretend that mmap() doesn't exist in ggc-page.c and fall back on
> valloc, then everything passes. But the comments say that is not the
> prefered solution. My question is: exactly what kind of penalty would
> I pay if I forced the use of valloc() versus using mmap?
It's OS-dependent. You should do some tests on your OS to find out.
Ideally, though, you would find some chunk of address space that is
always free on your OS and just use that with mmap; that's pretty much
guaranteed to be faster and more reliable.