[LAU] Isolating/dedicating Interrupts

[Robin Gareus]

On 11/22/2015 10:34 AM, Peter P. wrote:
> * Len Ovens <len at ovenwerks.net> [2015-11-21 18:55]:
> [...]
>> I would probably turn HT off anyway and the i5 runs cooler
>> (=quieter BTW with similar cooling setup) If I wanted more speed, I would
>> look at a xeon with no HT or an i7 and turn ht off in BIOS if I could. That
>> is I would add more cores rather than HT.

You can also tell the kernel with 'noht' to not use hyperthread features
of the CPU, or disable processors at runtime.

> Interesting, Len, why do you recommend to disable HyperThreading? What
> problems / disadvantages wrt. audio could it cause? 

There is anecdotal evidence that HT increases processing latency.

In short: Hyper-threading is one-CPU core that can run two threads.

Some of the *basic* CPU infrastructure is duplicated for every core, but
the FPU is not, nor are the L2, L3 CPU caches (this depends on the model).

In theory instead of e.g. waiting for I/O, or while doing on FPU
operations the same CPU core could do other tasks (e.g. integer calc,..)
if HT is enabled.

In practice most pro-audio tasks are float operations and there's a only
single FPU per core. To the OS it looks like there are two CPUs though.
If two cpu-threads need the same FPU one stalls and there's no benefit.

In theory this should be not worse than doing the two tasks sequentially
and HT can be better in some cases. In practice that's not always the
case, likely because the caches are shared.

The whole thing is yet another complex system where the only way for a
linux-audio-user to really know if it makes sense to use HT is to just
try an measure.

best,
robin

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