The x32 system call ABI [LWN.net]

The x32 system call ABI

Posted Sep 7, 2011 6:23 UTC (Wed) by butlerm (guest, #13312)
In reply to: The x32 system call ABI by gmaxwell
Parent article: The x32 system call ABI

If x32 compiled distributions run significantly faster than x64, it seems rather likely to me that desktop users will generally end up with _one_ x32 copy of system libraries in memory, with x64 libraries only loaded for the occasional application that needs a very large memory space.

With open source applications, what is there to complain about? If you don't like x32 just use x64 only.

And of course the big advantage of x32 over pointer compression is that no source modifications are required, modifications that in a typical C application would be extremely painful.

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The x32 system call ABI

Posted Sep 7, 2011 6:47 UTC (Wed) by gmaxwell (guest, #30048) [Link] (2 responses)

"If you don't like x32 just use x64 only" which means I get to go back to the bad old days of playing (int) to (void *)/(size_t) conversion guy because when 64 bit systems weren't commonly deployed on developers desktops a lot of stuff simply didn't work without a bunch of fuss. The freedom of open source has tremendous but not infinite value there is a real cost to being an oddball.

"If x32 compiled distributions run significantly faster than x64" IFF, but based on the currently available micro-benchmarks this seems unlikely. I've yet to see an example of a single application which is faster in x32 than best_of(x86,x86_64), and if we're in the two libraries mode then taking the choice of x86 for those few memory bandwidth bound pointer heavy apps that don't mine the scalability constraint is no worse.

"occasional application" like... my browser? (which is currently using ~4GiB of VM, though not as much resident obviously).

Not to mention the reduced address space for ASLR.

The x32 system call ABI

Posted Sep 8, 2011 21:19 UTC (Thu) by JanC_ (subscriber, #34940) [Link]

It's using almost 4 GiB on a 64-bit system now? But of course your browser would supposedly need significantly less memory when running in x32 mode? And once Firefox also uses out-of-process rendering (like Chrome/Chromium), that would become even less of an issue...?

The x32 system call ABI

Posted Sep 9, 2011 2:30 UTC (Fri) by butlerm (guest, #13312) [Link]

>I've yet to see an example of a single application which is faster in x32 than best_of(x86,x86_64)

That is the wrong metric to judge an ABI by - unless you agree that we should stick with an x86 + x86_64 biarchy indefinitely, and have distributions compile every other application appropriately. Then we really will end up with both sets of libraries pinned in memory.

x32 is noticeably better than x86, on some benchmarks as much as 30% more. It is also noticeably better than x86_64, another 30% on important workloads. It is a better all around ABI for most applications.

x86 is stunted, and will hopefully go away in a few years. But x32 sounds like it is worth keeping around for a long time. A 30% performance increase on many workloads isn't the sort of thing you want to idly throw away.