On Mon, Aug 19, 2013 at 10:00 PM, Greg Kroah-Hartman
<[email protected]> wrote:
> On Mon, Aug 19, 2013 at 07:04:18PM +0800, Ming Lei wrote:
>> Because usb_hcd_submit_urb is in the hotest path of usb core,
>> so use percpu counter to count URB instead of using atomic variable
>> because atomic operations are much slower than percpu operations.
>>
>> Cc: Oliver Neukum <[email protected]>
>> Cc: Alan Stern <[email protected]>
>> Signed-off-by: Ming Lei <[email protected]>
>> ---
>> drivers/usb/core/hcd.c | 4 ++--
>> drivers/usb/core/sysfs.c | 7 ++++++-
>> drivers/usb/core/usb.c | 9 ++++++++-
>> drivers/usb/core/usb.h | 1 +
>> include/linux/usb.h | 2 +-
>> 5 files changed, 18 insertions(+), 5 deletions(-)
>
> And this really speeds things up? Exactly what does it?
>
> And it's not that atomic operations are "slower", it's just that the
For SMP, atomic_inc/atomic_dec are much slower than percpu
variable inc/dec, see 4.1(Why Isn’t Concurrent Count-ing Trivial?)
of [1].
However, it is slower: on a Intel Core Duo laptop, it is about six
times slower than non-atomic increment when a single thread
is incrementing, and more than ten times slower if two threads
are incrementing.
Considered that most of desktop & laptop are SMP now, and with
USB3.0, the submitted URBs per second may reach tens of thousand
or more, and we can remove the atomic inc/dec operations in the hot
path, so why don't do it?
> barriers involved can be slower, depending on what else is happening.
> If you look, you are already hitting atomic variables in the same path,
> so how can this change speed anything up?
No, no barriers are involved in atomic_inc/atomic_dec at all.
[1], Is Parallel Programming Hard, And, If So, What Can You
Do About It?
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git
Thanks,
--
Ming Lei
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