64-bit performance [LWN.net]

64-bit performance

Posted Apr 15, 2008 4:39 UTC (Tue) by JoeBuck (subscriber, #2330)
In reply to: 64-bit performance by epa
Parent article: My kid hates Linux (ZDNet)

Take a box with 2 Gb of memory. Run a program that requires a 1.5 Gb working set to avoid paging with 32 bit code, where the in-memory structure is heavy with pointers. Now recompile with -m64. Voila, it now needs maybe 2.5 Gb to avoid paging. You might well see the 64 bit code run 100 times slower.

We run workloads like that all the time.

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64-bit performance

Posted Apr 15, 2008 6:08 UTC (Tue) by bronson (subscriber, #4806) [Link]

I suppose it's true that if you use an artificial 2.5 GB dataset and impose a 2 GB memory
limit, 32 bit would be faster.

In the real world, why wouldn't you just spend $50 for a 2GB memory upgrade?  Then the 64 bit
box would fly.  If you're not convinced, let's try this exercise again with a hypothetical 3.2
GB data set.  :)

In my experience, modern 64 bit boxes with stuffed with lots of ram are really cheap and
really damn fast.  I can't think of any reason to deploy 32 bit for servers/HPC these days.

64-bit performance

Posted Apr 15, 2008 18:38 UTC (Tue) by JoeBuck (subscriber, #2330) [Link]

"If you're not convinced, let's try this exercise again with a hypothetical 3.2 GB data set."

You've just answered your own question. Now you can run a problem that requires a 3.2GB working set with your 32 bit executable quickly (if you can squeeze it into the 4GB address space, and you'll need what Red Hat used to call the hugemem patch to make it work), but it takes maybe 5GB with the 64 bit executable, and the 32 bit version runs quicker.

Of course, you need the 64 bit executable when the problem size exceeds 4GB. The point is, it is useful for the developer to provide the user with both executables, to run on an operating system that can run both.