Andi Kleen wrote:
Good question. Btw. - faster by what order of magnitude?
pushf + popf is on K8 at least ~18 cycles, on P4 it is much more
because they synchronize the pipeline there (hundreds of cycles)
cpu local add would be a few cycles at best and doesn't have
any impact on the pipeline
local_irq_save/restore seems to be fine for kernel/profile.c
Reason 1:
cpu_local_* uses __get_cpu_var, which conflicts with struct statistic
being embedded into struct xyz that is allocated whenever the client
needs it.
I could try to use local_t in conjunction with local_add etc.
(as seen in include/linux/dmaengine.h in 2.6.17-mm6).
Does this also yield a performance gain worth consideration?
Yes, but you would need preempt_disable() then. For non preemptible
kernels (far majority) that would be already a big win.
So, removing local_irq_save/restore would require statistics to be
switched on and their buffers being available all the time. That is,
buffers holding counters etc. can't be allocated at run time - what
if allocation fails? (Should I leave this issue to clients?).
Can't you use RCU for this?
Reason 4:
The alleged overhead of local_irq_save/restore (as compared
to atomic operations)
local_* doesn't need to be atomic. IT isn't on x86 at least.
On some other architectures it can be, but i think it's just a SMOP
of fixing them.
-Andi
Thanks. I am seriously considering these techniques.
If I manage to use RCU for most of the read-mostly struct statistic
and to push any other locking issues down into individual statistic
disciplines (utilisation indicator, histogram and so on), I should be
able to use local_t for some disciplines (particularyl counter and
histogram) without needing other locking primitives, like
local_irq_save/restore currently currently found in the code.
Not a change that can be done this afternoon, though.
And it will require careful review.
Martin
-
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]
