From guent...@gmail.com Sun Oct 26 17:01:26 2014
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References: <20141026190245.b09d21b5...@wolfman.devio.us>
Subject: Re: 64-bit amd64 : actual memory limitations?
From: Philip Guenther <guent...@gmail.com>
To: Mayuresh Kathe <mayur...@devio.us>
Cc: OpenBSD misc <misc@openbsd.org>
Content-Type: text/plain; charset=UTF-8
On Sun, Oct 26, 2014 at 12:02 PM, Mayuresh Kathe <mayur...@devio.us>
wrote:
> 64-bit supposedly supports upto 16 exabytes of memory ('ram').
> would such large capacities actually be possible to ue with
> openbsd for amd64 architecture?
> use-case: working with large in-memory storage for financial
applications.
Currently, in the amd64 pmap we only allocate a single PML4 slot for
the 'direct' map, limiting the system to 512GB of physical memory.
Multiplying that by a factor of 4 or even 64 would be easy, but
lacking someone actually planning to *use and test* that it just
hasn't been seen as worth the effort to do, verify the change in kva
layout doesn't break some hidden assumption, etc. (IIRC, dlg@ did
this once just to get a dmesg from a 2TB machine going to other,
non-OpenBSD use.)
There's an upper limit to that growth: there's only 256 PML4 slots for
kva, from which this comes, so at some point the kva remaining for
normal use and management would shrink too much and the whole pmap
would need to be rethought...
Philip Guenther
thanks for the details, but, since i am nowhere close to being a
systems programmer, would like to know if there's any chance such
support might be added in, in the near future?
thanks.
