[mythtv-users] [mythtv-commits] Ticket #11539: New capture card Hauppauge WinTV-HVR-950Q FE_GET_INFO No Such Device

[Raymond Wagner]

From: Jean-Yves Avenard
Sent: Tuesday, May 14, 2013 9:19 AM
> The fact is. Not all processors are equal with 64 bits mode.
>
> A core 2 duo, running a 64 bits kernel *is* slower than a 32 bits one.
> Not so long ago, got a few reports that a 0.25 version of myth wasn't
> playing a file well, when 0.24 was just fine. This was on a mac.
> Turned out, by default I had made the app use 64 bits; because hey, 64
> is at least twice better than 32 :)

> Turned out, that's true on i3,i5 and i7, but not so on core 2 duo.

There are some performance deficits when dealing with 64-bit applications,
as a direct result of them being 64-bit. Many things are now immediately
twice the size, which means for the same operation, you have twice the
memory IO, and twice the cache usage, resulting in potential issues with
cache coherency. Still, the advantages typically outweigh those deficits,
with the improvement from an expanded instruction set and register space
accounting for the losses in memory and cache.

My primary point was still more with regards to reliability. There have
been many instances in the past few years where problems were caused
simply by running 32-bit. Two that come to mind off the top of my head
are vmalloc and autoexpirer overflow. nVidia drivers eat up a lot of
kernel virtual address space, and so can cause problems when used in
combination with tuners and other devices that similarly want that space.
On 64-bit Linux, the default allocation is significantly larger, and that
issue does not manifest. The autoexpirer was set up to dangerously use a
standard integer to store free space. On a 64-bit system, everything works
just fine. On a 32-bit system, once you hit 2TB free on a partiton, the
integer rolls over, and the autoexpirer goes nuts, deleting everything
it can get its hands on until it reaches 4TB and rolls over again.

32-bit x86 systems simply don't get the eyes on them that 64-bit x86_64
systems do, and thus there will always be more problems such as these.