From: Glenn Matthys - OpenGate (glennopengate.be)
Date: Thu Aug 02 2007 - 06:05:41 CDT

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Michael Tokarev wrote:
> Adam Jacob Muller wrote:
>> As an aside, if you are upgrading the same machine to 64-bit with the
>> same hardware and expecting better performance, you will not find this.
>
> Please excuse me for somewhat off-topic here, but I want to comment on this.
>
> The thing is - whenever after switching to 64bits one will see any
> speedups or slowdowns really depends on the workload and other issues.
>
> There are 2 components involved - kernel, which may be 64 or 32 bits,
> and userspace (including postfix itself), which, with 64bits kernel,
> also may be 64 or 32 bits.
>
> In kernel, there are alot of places where 64bits arithmetics is used.
> Including many filesystem (I/O) operations, timekeeping and so on.
> Those things are running faster on 64bits (i.e. without software
> emulation of all the arith operations). This one fact alone may
> speed up some workloads.
>
> x86-64 systems has more registers than traditional x86. Which speeds
> up things further, sometimes significantly -- especially computation-
> intensive tasks.
>
> There are addressing problems in 32bit mode as well - for example,
> with, say, 2Gb memory, kernel has to do some work to ensure that
> DMA areas stays below 1Gb anyway due to hardware limitations, thus
> wasting "low" memory for DMA, and moving/remapping userspace pages
> on direct I/O for example.
>
> There are other improvements in x86-64 compared to x86 as well.
>
> And there are several downsides: word size is increased in 64bits,
> so is memory requirements (one now needs twice the memory for
> pointers and "normal" integers). With mixed 64bits kernel and
> 32bits userspace we may be wasting additional time on syscall
> conversions inside the kernel.
>
> Postfix itself does not benefit from 64bits. But the system
> as a whole may. Again, with postfix-only workload even if
> there will be some benefits, they will barely be measurable/
> noticeable. If the same system is doing something else too,
> it may become another story.
>
> /mjt

I can confirm this. In my experience, machines that are running a fully
native 64 bit environment can cope with more load and stay more
responsive under stressfull conditions.

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