Re: DSA on 32 bit systems

On Wed, Jul 12, 2017 at 7:34 AM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> That raises the question of whether we should go further and use a 64
> bit dsa_pointer even on 32 bit systems. In a 32 bit dsa_pointer
> build, a single dsa_area is limited to 32 segments of up to 128MB in
> size (actually the largest power of 2 size it can allocate at once is
> 64MB due to free page management overhead). The 64 bit version can
> handle 1024 segments of up to 1TB, which ought to be enough for
> anybody. Having just one build variant without any limits you're
> likely to hit is appealing, but using oversized pointers will
> presumably slow 32 bit systems down, perhaps especially for client
> code that uses dsa_pointer_atomic (though I haven't tested that).

Hmm. What we do we think the largest single allocation that has a
chance of working on a 32-bit system is in general? My guess is that
1GB is not likely to work unless things break very well for you, but
256MB and 512MB might, absent implementation limitations.

> If we keep 32 bit dsa_pointer, I now see that parallel hash should
> probably know about this limit so it can abandon its load factor goal
> rather than making a doomed allocation request.

My overall feeling here is that this *probably* doesn't matter much
one way or the other, and we don't really have enough information
right now to know whether wider dsa_pointers are a bigger problem than
not being able to make large allocations, or visca versa. But count
me as +0 for widening them to 64-bits.

--
Robert Haas
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