Hi Willy,
I think at least I can remove the LOCK instruction when the lock is already
held by someone else and enter the spinning wait directly, right?
0: cmpb $0, slp
jle 2f # lock is not available, then spinning
directly without locking the bus
1: lock; decb slp # lock the bus and atomically decrement
jns 3f # if clear sign bit jump forward to 3
2: pause # spin - wait
cmpb $0,slp # spin - compare to 0
jle 2b # spin - go back to 2 if <= 0 (locked)
jmp 1b # unlocked; go back to 1 to try to lock again
3: # we have acquired the lock .
But based on the Lockmeter report, the lock success is dominant 99.8%, so
maybe this will not make much change.
Thanks,
Liang
----- Original Message -----
From: "Willy Tarreau" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Wednesday, July 13, 2005 10:16 PM
Subject: Re: About a change to the implementation of spin lock in 2.6.12
kernel.
Hi,
On Wed, Jul 13, 2005 at 07:20:06PM -0700, [email protected]
wrote:
Hi,
I found _spin_lock used a LOCK instruction to make the following
operation "decb %0" atomic. As you know, LOCK instruction alone takes
almost 70 clock cycles to finish and this add lots of cost to the
_spin_lock. However _spin_unlock does not use this LOCK instruction and
it uses "movb $1,%0" instead since 4-byte writes on 4-byte aligned
addresses are atomic.
_spin_unlock does not need locked operations because when it is run, the
code is already known to be the only one to hold the lock, so it can
release it without checking what others do.
So I want rewrite the _spin_lock defined spinlock.h
(/linux/include/asm-i386) as follows to reduce the overhead of _spin_lock
and make it more efficient.
It does not work. You cannot write an inter-cpu atomic test-and-set with
several unlocked instructions.
#define spin_lock_string \
"\n1:\t" \
"cmpb $0,%0\n\t" \
"jle 2f\n\t" \
==> here, another thread or CPU can get the lock simultaneously.
"movb $0, %0\n\t" \
"jmp 3f\n" \
"2:\t" \
"rep;nop\n\t" \
"cmpb $0, %0\n\t" \
"jle 2b\n\t" \
"jmp 1b\n" \
"3:\n\t"
Compared with the original version as follows, LOCK instruction is
removed. I rebuilt the Intel e1000 Gigabit driver with this _spin_lock.
There is about 2% throughput improvement.
#define spin_lock_string \
"\n1:\t" \
"lock ; decb %0\n\t" \
"jns 3f\n" \
"2:\t" \
"rep;nop\n\t" \
"cmpb $0,%0\n\t" \
"jle 2b\n\t" \
"jmp 1b\n" \
"3:\n\t"
Do you think I can get a better performance if I dig further?
Any ideas will be greatly appreciated,
well, of course with those methods you can improve performance, but you
lose the warranty that you're alone to get a lock, and that's bad.
another similar method to get a lock in some very controlled environment
is as follows :
1: cmp $0, %0
jne 1b
mov $CPUID, %0
membar
cmp $CPUID, %0
jne 1b
This only works with same speed CPUs and interrupts disabled. But in
todays
environments, this is very risky (hyperthreaded CPUs, etc...). However,
this
is often OK for more deterministic CPUs such as microcontrollers.
Regards,
Willy
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