Jarek Poplawski wrote:
On Fri, Oct 12, 2007 at 10:42:34AM +0200, Helge Hafting wrote:
Jarek Poplawski wrote:
On 04-10-2007 07:23, Nick Piggin wrote:
According to latest memory ordering specification documents from Intel and
AMD, both manufacturers are committed to in-order loads from cacheable
memory
for the x86 architecture. Hence, smp_rmb() may be a simple barrier.
...
Great news!
First it looks like a really great thing that it's revealed at last.
But then... there is probably some confusion: did we have to use
ineffective code for so long?
You could have tried the optimization before, and
gotten better performance. But if without solid knowledge that
the optimization is _valid_, you risk having a kernel
that performs great but suffer the occational glitch and
therefore is unstable and crash the machine "now and then".
This sort of thing can't really be figured out by experimentation, because
the bad cases might happen only with some processors, some
combinations of memory/chipsets, or with some minimum
number of processors. Such problems can be very hard
to find, especially considering that other plain bugs also
cause crashes.
Therefore, the "ineffective code" was used because it was
the only safe alternative. Now we know, so now we may optimize.
Sorry, I don't understand this logic at all. Since bad cases
happen independently from any specifications and Intel doesn't
take any legal responsibility for such information, it seems we
should better still not optimize?
The point is that we _trust_ intel when they says "this will work".
Therefore, we can use the optimizations. It was never about
legal matters. If we didn't trust intel, then we couldn't
use their processors at all.
We couldn't take the chance before. It was not documented
to work, verification by testing would not be trivial at all for
this case.
Linux is about "stability first, then performance".
Now we _know_ that we can have this optimization without
compromising stability. Nobody knew before!
Helge Hafting
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
