kernel defconfig, debugging, preemption, and very noticeable speedups/ debugging

Werner Almesberger werner at
Sun Jan 10 05:52:22 CET 2010

Paul Fertser wrote:
> I suspect that the most useful debugging features (that should be in
> kernels all users are using) add no overhead at all.

That's also my expectation. Therefore, I'm a bit surprised by the
massive improvements reported.

Now, it could well be that there are some debug options that look
perfectly harmless (at least to me) but that have a high cost.

Another possibility would be that some options just produce a
small increase of the amount of kernel messages printed. That in
turn could, combined with the very inefficient scrolling to the
kernel's frame, buffer cause significant effects in some tests.

Another possibility is that some systems may encounter a large
number of slightly abnormal conditions that generate messages when
debugging is enabled, while other systems are quiet. Thus, not all
developers may experience the slowdown.

So, something was found, but I think a bit more work is needed to
identify what it really is. Identifying the costly debug options
will also help to be aware of the performance impact if they are
needed in the future.

I would agree that many debug options are indeed only useful when
chasing a bug, but some also produce diagnostics useful for
getting a first idea of what's happening. We would win little if
users would normally run an extremely lean kernel but get asked to
install some fat debug kernel each time they report a bug.

Related to this, it would be great if the GTA02 kernel's text
console had Glamo-accelerated scrolling. A long time ago, I
thought adding such a block move would be a nice late night
project, but I failed to elicit even the slightest response from
the Glamo. Some time later, Harald mentioned that he was giving it
a try as well, but apparently without much success either. Maybe
third time's lucky ?

By the way, Gennady, your mails may not reach some readers because
they're dated way into the future (about 9 months ahead), which is
something that normally identifies spam. And even for those who do
see the mails, it messes up their mailbox order.

- Werner

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