Re: hdparm -t yields incorrect timings when Intel hyperthreading is enabled
On Fri, 09 May 2014, Paul Ausbeck wrote:
> Henrique, thanks a lot for the detailed reply. I will look at the
> stuff that you suggested, if only to learn about what I don't know.
>
> FYI, the problem doesn't seem related to temperature to me. I'm not
> ruling it out, I'm just saying it doesn't have that feel.
What I mean is that when you mess with the low-level idle loop driver, it is
not impossible to get all cores and hardware threads of the CPU stuck at C0
state (i.e. running 100% of the time). This *shoudln't* happen, but still
it is best that you are aware of the possibility that you might end up
testing the cpu heatsink/cooler as well...
> But, the system doesn't appear to be THAT much more unresponsive
> when hyperthreading is enabled over when disabled. So I'm leaning
> toward the idea that hdparm's calculation is being spoofed somehow.
Hmm, there's a way to test. Time how much wall-clock time it takes to write
a large file to disk and sync (flush to disk) the result, preferably in
single-user mode:
sync; date ; dd if=/dev/zero of=somefile bs=10M count=100 ; sync; date
That should do it (the above creates an 1GiB file). It will also test the
scheduler, as that /dev/zero read does cause syscalls and switches to kernel
context.
BTW, the output of "hdparm -T" should not vary an order of magnitude. If it
does and there is nothing obnoxious running in the background, you've got
closer to the problem. I doubt it is something like this, though: your
system would be sluggish as all heck...
> Lastly, the difference in boot times may not be due to the
> hyperthreading issue, at least directly. There are quite a few
Indeed. However, when looking at the timings in the /var/log/dmesg
comparison, remember the comparison is less for the timings, and more for
everything else. Note that some reordering across boots, even of the same
kernel, is perfectly normal.
--
"One disk to rule them all, One disk to find them. One disk to bring
them all and in the darkness grind them. In the Land of Redmond
where the shadows lie." -- The Silicon Valley Tarot
Henrique Holschuh
Reply to: