Posted Apr 17, 2004 3:40 UTC (Sat) by RichardRudell
Parent article: The Grumpy Editor goes 64-bit
Wow. Lots of feedback on what should now be an old topic (-:
Redhat Enterprise Linux 3 was my choice and it works flawlessly. Pricey, yes, but all of the features needed for compute-intensive applications which need more than 32-bits. Robust, solid, and supported by the third-party software I need.
I notice that LWN has never done a piece on how RHEL3, which is a 2.4 kernel, has all of the most useful features of the 2.6 kernel backported to a solid and stable platform. If you need me to list what they are, perhaps a detailed piece analyzing the RHEL3 kernel is more in order ...
[I have no relationship to RedHat other than being a satisfied customer].
BTW: it's not 64-bit addressing. Hypertransport has a physical limitation of 40-bits. Yeah, I couldn't believe it either until I read the specs. 1 TB is a nice improvement over 4 GB, but it's not the 2^64 talked about in an earlier post. Forget virtual address space (currently 48-bits on Opteron), I'm talking about physical address space. There is a hardwired limit which is going to hurt sooner than later.
Worse, because "int" is still 32-bit, we should really call this the "36-bit" machine, i.e., many applications will break because they count the "number of things" using an "int"; if the "thing" is 32-bytes, the "int" overflows at 2^36 bytes. That's only 16 GB which is too near current reality for my liking.
Also, 64-bit mode beats 32-bit mode on Opteron only for "small memory" applications. "Big memory" applications, i.e., why you want 64-bits in the first place, still lose by 5-20%. See, the problem is pointers being 64-bits reduces the effective size of the L1/L2 cache and cause increased traffic to DDR. Typical applications show a 1.6x increase in memory space (each struct being a mix of pointers (64-bit) and ints (32-bit). The extra 8 registers and the new ABI (pass arguments in registers, no frame pointer) help, but you just can't overcome a 60% increase in memory traffic when you really need to use all that memory.
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