Hello Andrea,
Interesting benchmarks... did you compile the test programs with "make
SCHED=yes" by any chance? Also what other software are you running?
The reason I ask is that running a full blown KDE setup running in the
background, I get the following numbers on the rwsem-ro test (XADD optimised
kernel):
SCHED: 4615646, 4530769, 4534453 and 4628365
no SCHED: 6311620, 6312776, 6327772 and 6325508
Also quite stable as you can see.
> (ah and btw the machine is a 2-way PII 450mhz).
Your numbers were "4274607" and "4280280" for this kernel and test This I
find a little suprising. I'd expect them to be about 10% higher than I get on
my machine given your faster CPUs.
What compiler are you using? I'm using the following:
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-80)
Something else that I noticed: Playing a music CD appears to improve the
benchmarks all round:-) Must be some interrupt effect of some sort, or maybe
they just like the music...
> rwsem-2.4.4-pre6 + my new generic rwsem (fast path in C inlined)
Linus wants out of line generic code only, I believe. Hence why I made my
generic code out of line.
I have noticed one glaring potential slowdown in my generic code's down
functions. I've got the following in _both_ fastpaths!:
struct task_struct *tsk = current;
It shouldn't hurt _too_ much (its only reg->reg anyway), but it will have an
effect. I'll have to move it and post another patch tomorrow.
I've also been comparing the assembly from the two generic spinlock
implementations (having out-of-lined yours in what I think is the you'd have
done it). I've noticed a number of things:
(1) My fastpaths have slightly fewer instructions in them
(2) gcc-2.96-20000731 produces what looks like much less efficient code
than gcc-snapshot-20010409 (to be expected, I suppose).
(3) Both compilers do insane things to registers (like in one instruction
moving %eax to %edx and then moving it back again in the next).
(4) If _any_ inline assembly is used, the compiler grabs extra chunks of
stack which it does not then use. It will then pop these into registers
under some circumstances. It'll also save random registers it doesn't
clobber under others.
(Basically, I had a lot of frustrating fun playing around with the spinlock
asm constraints trying to avoid the compiler clobbering registers
unnecessarily because of them).
I've attached the source file I've been playing with and an example
disassembly dump for your amusement. I used the snapshot gcc to do this (it
emits conditional chunks of code out of line more intelligently than the
older one.
It's also interesting that your generic out-of-line semaphores are faster
given the fact that you muck around with EFLAGS and CLI/STI, and I don't.
Maybe I'm getting hit by an interrupt. I'll have to play around with it and
benchmark it again.
David
--------------Boundary-00=_H7U7UYK0T7DOAVAKJABQ
Content-Type: text/x-plain;
charset="iso-8859-1";
name="slowpath.c"
Content-Transfer-Encoding: 8bit
Content-Description: rwsem implementations slowpath comparison C source code
Content-Disposition: attachment; filename="slowpath.c"
/* slowpath.c: description
*
* Copyright (c) 2001 David Howells (dhowells@redhat.com).
*/
#define __KERNEL__
#include <asm/types.h>
#include <asm/current.h>
#include <asm/system.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/spinlock.h>
struct rw_semaphore_dwh {
__u32 active;
__u32 waiting;
spinlock_t wait_lock;
};
extern void FASTCALL(dwh_down_read(struct rw_semaphore_dwh *));
extern void FASTCALL(dwh_up_read(struct rw_semaphore_dwh *));
extern void FASTCALL(dwh_down_read_failed(struct rw_semaphore_dwh *,
struct task_struct *));
extern void FASTCALL(dwh__rwsem_do_wake(struct rw_semaphore_dwh *));
struct rw_semaphore_aa {
spinlock_t lock;
int count;
struct list_head wait;
};
#define RWSEM_WAITQUEUE_READ 0
extern void FASTCALL(aa_down_read(struct rw_semaphore_aa *));
extern void FASTCALL(aa_up_read(struct rw_semaphore_aa *));
extern void FASTCALL(aa_down_failed(struct rw_semaphore_aa *, int));
extern void FASTCALL(aa_rwsem_wake(struct rw_semaphore_aa *));
void dwh_down_read(struct rw_semaphore_dwh *sem)
{
struct task_struct *tsk = current;
spin_lock(&sem->wait_lock);
if (sem->waiting) {
sem->active++;
spin_unlock(&sem->wait_lock);
goto out;
}
sem->waiting++;
dwh_down_read_failed(sem,tsk);
spin_unlock(&sem->wait_lock);
out:
}
void aa_down_read(struct rw_semaphore_aa *sem)
{
spin_lock_irq(&sem->lock);
if (sem->count < 0 || !list_empty(&sem->wait))
goto slow_path;
sem->count++;
out:
spin_unlock_irq(&sem->lock);
return;
slow_path:
aa_down_failed(sem, RWSEM_WAITQUEUE_READ);
goto out;
}
void dwh_up_read(struct rw_semaphore_dwh *sem)
{
spin_lock(&sem->wait_lock);
if (--sem->active==0 && sem->waiting)
dwh__rwsem_do_wake(sem);
spin_unlock(&sem->wait_lock);
}
void aa_up_read(struct rw_semaphore_aa *sem)
{
unsigned long flags;
spin_lock_irqsave(&sem->lock, flags);
if (!--sem->count && !list_empty(&sem->wait))
aa_rwsem_wake(sem);
spin_unlock_irqrestore(&sem->lock, flags);
}
--------------Boundary-00=_H7U7UYK0T7DOAVAKJABQ
Content-Type: text/plain;
charset="iso-8859-1";
name="slowpath.dis"
Content-Transfer-Encoding: 8bit
Content-Description: disassembly of compiled slowpath.c
Content-Disposition: attachment; filename="slowpath.dis"
slowpath.o: file format elf32-i386
Disassembly of section .text:
00000000 <dwh_down_read>:
0: 53 push %ebx
1: 83 ec 08 sub $0x8,%esp
4: 89 c3 mov %eax,%ebx
6: ba 00 e0 ff ff mov $0xffffe000,%edx
b: 21 e2 and %esp,%edx
d: f0 fe 4b 08 lock decb 0x8(%ebx)
11: 0f 88 fc ff ff ff js 13 <dwh_down_read+0x13>
17: 8b 4b 04 mov 0x4(%ebx),%ecx
1a: 85 c9 test %ecx,%ecx
1c: 74 0a je 28 <dwh_down_read+0x28>
1e: ff 03 incl (%ebx)
20: c6 43 08 01 movb $0x1,0x8(%ebx)
24: 58 pop %eax
25: 5a pop %edx
26: 5b pop %ebx
27: c3 ret
28: c7 43 04 01 00 00 00 movl $0x1,0x4(%ebx)
2f: 89 d8 mov %ebx,%eax
31: e8 fc ff ff ff call 32 <dwh_down_read+0x32>
36: eb e8 jmp 20 <dwh_down_read+0x20>
00000038 <aa_down_read>:
38: 53 push %ebx
39: 83 ec 08 sub $0x8,%esp
3c: 89 c3 mov %eax,%ebx
3e: fa cli
3f: f0 fe 0b lock decb (%ebx)
42: 0f 88 09 00 00 00 js 51 <aa_down_read+0x19>
48: 8b 53 04 mov 0x4(%ebx),%edx
4b: 85 d2 test %edx,%edx
4d: 78 19 js 68 <aa_down_read+0x30>
4f: 8d 43 08 lea 0x8(%ebx),%eax
52: 39 43 08 cmp %eax,0x8(%ebx)
55: 75 11 jne 68 <aa_down_read+0x30>
57: 8d 42 01 lea 0x1(%edx),%eax
5a: 89 43 04 mov %eax,0x4(%ebx)
5d: c6 03 01 movb $0x1,(%ebx)
60: fb sti
61: 5b pop %ebx
62: 58 pop %eax
63: 5b pop %ebx
64: c3 ret
65: 8d 76 00 lea 0x0(%esi),%esi
68: 31 d2 xor %edx,%edx
6a: 89 d8 mov %ebx,%eax
6c: e8 fc ff ff ff call 6d <aa_down_read+0x35>
71: eb ea jmp 5d <aa_down_read+0x25>
73: 90 nop
00000074 <dwh_up_read>:
74: 53 push %ebx
75: 83 ec 08 sub $0x8,%esp
78: 89 c3 mov %eax,%ebx
7a: f0 fe 4b 08 lock decb 0x8(%ebx)
7e: 0f 88 15 00 00 00 js 99 <dwh_up_read+0x25>
84: 8b 03 mov (%ebx),%eax
86: 48 dec %eax
87: 89 03 mov %eax,(%ebx)
89: 85 c0 test %eax,%eax
8b: 74 0b je 98 <dwh_up_read+0x24>
8d: c6 43 08 01 movb $0x1,0x8(%ebx)
91: 5a pop %edx
92: 59 pop %ecx
93: 5b pop %ebx
94: c3 ret
95: 8d 76 00 lea 0x0(%esi),%esi
98: 8b 43 04 mov 0x4(%ebx),%eax
9b: 85 c0 test %eax,%eax
9d: 74 ee je 8d <dwh_up_read+0x19>
9f: 89 d8 mov %ebx,%eax
a1: e8 fc ff ff ff call a2 <dwh_up_read+0x2e>
a6: eb e5 jmp 8d <dwh_up_read+0x19>
000000a8 <aa_up_read>:
a8: 56 push %esi
a9: 53 push %ebx
aa: 51 push %ecx
ab: 89 c3 mov %eax,%ebx
ad: 9c pushf
ae: 5e pop %esi
af: fa cli
b0: f0 fe 0b lock decb (%ebx)
b3: 0f 88 22 00 00 00 js db <aa_up_read+0x33>
b9: 8b 43 04 mov 0x4(%ebx),%eax
bc: 48 dec %eax
bd: 89 43 04 mov %eax,0x4(%ebx)
c0: 85 c0 test %eax,%eax
c2: 74 0c je d0 <aa_up_read+0x28>
c4: c6 03 01 movb $0x1,(%ebx)
c7: 56 push %esi
c8: 9d popf
c9: 5a pop %edx
ca: 5b pop %ebx
cb: 5e pop %esi
cc: c3 ret
cd: 8d 76 00 lea 0x0(%esi),%esi
d0: 8d 43 08 lea 0x8(%ebx),%eax
d3: 39 43 08 cmp %eax,0x8(%ebx)
d6: 74 ec je c4 <aa_up_read+0x1c>
d8: 89 d8 mov %ebx,%eax
da: e8 fc ff ff ff call db <aa_up_read+0x33>
df: eb e3 jmp c4 <aa_up_read+0x1c>
Disassembly of section .text.lock:
00000000 <.text.lock>:
0: 80 7b 08 00 cmpb $0x0,0x8(%ebx)
4: f3 90 repz nop
6: 7e f8 jle 0 <.text.lock>
8: e9 09 00 00 00 jmp 16 <.text.lock+0x16>
d: 80 3b 00 cmpb $0x0,(%ebx)
10: f3 90 repz nop
12: 7e f9 jle d <.text.lock+0xd>
14: e9 3b 00 00 00 jmp 54 <aa_down_read+0x1c>
19: 80 7b 08 00 cmpb $0x0,0x8(%ebx)
1d: f3 90 repz nop
1f: 7e f8 jle 19 <.text.lock+0x19>
21: e9 76 00 00 00 jmp 9c <dwh_up_read+0x28>
26: 80 3b 00 cmpb $0x0,(%ebx)
29: f3 90 repz nop
2b: 7e f9 jle 26 <.text.lock+0x26>
2d: e9 ac 00 00 00 jmp de <aa_up_read+0x36>
--------------Boundary-00=_H7U7UYK0T7DOAVAKJABQ--
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