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Date:      Tue, 23 Apr 2002 21:40:11 -0700
From:      David Schultz <dschultz@uclink.Berkeley.EDU>
To:        Vallo Kallaste <kalts@estpak.ee>
Cc:        "Marc G. Fournier" <scrappy@hub.org>, Terry Lambert <tlambert2@mindspring.com>, freebsd-current@FreeBSD.ORG, freebsd-stable@FreeBSD.ORG
Subject:   Re: FreeBSD 4.5-STABLE not easily scalable to large servers ... ?
Message-ID:  <20020423214011.B3593@HAL9000.wox.org>
In-Reply-To: <20020423184534.GA30212@myhakas.estpak.ee>; from kalts@estpak.ee on Tue, Apr 23, 2002 at 09:45:34PM %2B0300
References:  <3CC4C683.F9AEF14E@mindspring.com> <20020423092909.N1721-100000@mail1.hub.org> <20020423184534.GA30212@myhakas.estpak.ee>

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Thus spake Vallo Kallaste <kalts@estpak.ee>:
> Userspace processes will allocate memory
> from UVA space and can grow over 1GB of size if needed by swapping.
> You can certainly have more than one over-1GB process going on at
> the same time, but swapping will constrain your performance.

It isn't a performance constraint.  32-bit architectures have 32-bit
pointers, so in the absence of segmentation tricks, a virtual address
space can only contain 2^32 = 4G locations.  If the kernel gets 3 GB
of that, the maximum amount of memory that any individual user process
can use is 1 GB.  If you had, say, 4 GB of physical memory, a single
user process could not use it all.  Swap increases the total amount of
memory that *all* processes can allocate by pushing some of the pages
out of RAM and onto the disk, but it doesn't increase the total amount
of memory that a single process can address.

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