Re: [PATCHES] Try again: S_LOCK reduced contention

Once upon a time Bruce Momjian wrote:

OK, here is my argument for inlining tas().

This output is for startup, a single query returning a single row result
on an indexed column, and shutdown. Guess who is at the top of the
list, tas(). Mcount() is a gprof internal counter, so it doesn't
"count". tas() is showing 0.01 cummulative seconds. Psql shows
wallclock time for the query at 0.13 seconds. That 0.01 is pretty
significant.

...

% cumulative self self total
time seconds seconds calls ms/call ms/call name
20.0 0.02 0.02 mcount (463)
10.0 0.03 0.01 5288 0.00 0.00 _tas [31]

As promised, I did a little testing to see what part of this overhead
is measurement error due to the nature of gprof and to see what the real
overhead of various spinlock implementations are. Here is what I learned.

Section 1. Summary

1.1. Spinlocks are pretty cheap.

Spinlocks are relatively cheap operations. The worst implementation I
tested took 0.34 microseconds for a spinlock roundtrip (take and release).
The current (in CVS as of May 98) code takes 0.28 microseconds. The best
hand tuned asm implementation took only 0.14 microseconds.

This is fast enough that I had to use a huge iteration count (100,000,000)
to get reasonably large run times.

Table 1.1 Overheads of spinlock roundtrip in microseconds

Test Case Time (usec) Notes

Original 0.14 S_LOCK in 6.3.2 (no backoff, asm)
InlineTas 0.15 Patch to be submitted (backoff, _inline_)
TasFunction2 0.20 Refined S_LOCK patch TAS as function.
MacroSLOCK 0.28 May 98 S_LOCK patch as in CVS

1.2. gprof doesn't work for small quick functions.

gprof introduces severe experimental error for small quick functions.
According to the gprof profile done by Bruce, 5288 tas calls took 0.1
second. That would require the spinlock roundtrips to take almost 19
microseconds each, not 0.28 microseconds. So in reality the 5288 calls
took only 0.0015 seconds, not 0.1 seconds.

So gprof has overestimated the cost of the spinlock by 68 to 1.

Perhaps the spinlock function is so short and quick compared to the
mcount overhead added to the function prolog that the overhead dominates
the measurement. gprof remains a useful tool for larger functions with
longer runtimes, but must be considered very suspect for tiny functions.

1.3 Function calls are pretty cheap or Macros may not save all that much.

The difference between the current (late May) macro version and the same
code removed to a separate function and called with three arguments was
only 0.06 microseconds. That is 60 nanoseconds for the argument passing,
the call, and the return.

I suspect that on the x86 "architecture" the limited number of registers
means that inline code has to save results to memory as it goes along
which may offset to some extent the overhead of the register saves for
a function call.

1.4 There are mysteries. Beware.

In some of the test cases there was significant timing variation from
run to run even though the test conditions were apparently identical.
Even more strangely, the variation in time was not random but appeared
to represent two different modes. And, the variation was itself repeatable.

Here are the raw times in CPU seconds from two different experiments each
run six consecutive times:

case 1: 49.81, 49.43, 40.68, 49.51, 40.68, 40.69
clusters about 40.7 and 49.5 seconds

case 2: 39.34, 29.09, 28.65, 40.34, 28.64, 28.64
clusters about 28.9 and 39.6 seconds

Note that the testrun times have a bimodal distribution with one group of
fast runs clustered tightly about one time and then a much slower group
clustered tightly about the second time. The difference between groups is
huge (about 25%) while the diffence within a group is very small (probably
within the measurement error.

I have no explanation for this variation. Possibly it is some interaction
of where the program is loaded and the state of the memory heirarchy, but
even this is hard to sustain. I would be very curious to hear of any
plausible explainations.

1.5 Timing very small functions in isolation with large counts is effective.

Notwithstanding the strange behavior in section 1.4, it is possible to
time differences in functions that amount to the addition or deletion of
one or two instructions. For example, the TasFunction and TasFunction2
cases below are almost but not quite identical yet show noticably
different runtimes.

1.6. New patch to follow.

The current S_LOCK and TAS() implementations (my patch of late May) are
slower than they need to be and cause more code bloat than they need to.
The bloat is caused by using a macro to inline a relatively complex bit
of code that is only used in the blocked lock case. I suspect the slowness
is caused at least partly by the macro as it requires more registers.

I have developed a new patch that separates out the lock available case
from the busywaiting case and that uses the GCC _inline_ facilty to make
the asm interface still look as clean as a function while not costing
anything. For a preview, see

Section 2. Test Procedure

My test takes and releases a spinlock 100,000,000 times and measures the
total CPU time. I ran this test with many variations of spinlock
implementation. I also ran a baseline test that has just the loop and call
overheads with no actual spinlock so that we can separate out just the S_LOCK
time. The test harness code appears below and the variant spinlock
implementations, generated assembler output and raw result timings appear
later in this message.

Testing was done on "leslie" (P133 HX chipset 512K L2) running Linux 2.0.33.
The system was up and running X but no other workloads. I avoided typing
or touching the mouse during the test. Each variation was run three times
and the results averaged. For some tests there was significant variation in
times for the three iterations. In this case another set of three was run
and the average of six runs used.

Section 2.1 Test Harness Code

/*
* Test s_lock timing with variations
*/
typedef unsigned char slock_t;
volatile slock_t the_lock;

int main(void)
{
int i = 0;

the_lock = 0;
while (i < 100000000) { /* 100 million iterations */
i = tryit(&the_lock, &i); /* take and release spinlock */
}
return i & 1;
}

/*
* Take and release lock
*/
int tryit(volatile slock_t *lock, int *p)
{
int i;
S_LOCK(lock);
i = ++(*p); /* just to make it more realistic */
S_UNLOCK(lock);
return *p;
}

Section 3.0 Test Case Summary

Case Tag Per Iteration Comments
============= ============= ============================================
TZero: 0.14 usec compare lock to 0, do nothing
TZeroNoCall 0.17 usec compare lock to 0, call s_lock if not

TZeroCall 0.30 usec call function that compares lock to 0
TasFunction 0.45 usec lock spinlock in separate tas() function
TasFunction2 0.37 usec improved separate tas function
SlockAsmMacro 0.31 usec Inline xchgb in S_LOCK macro, call s_lock if
needed. Note strange variation in recorded
times. I have no explaination.
Original 0.31 usec The original S_LOCK from 6.3.2
MacroSLOCK 0.45 usec Current in CVS as of 5/30/98. Tends to bloat.
AllFunctions 0.51 usec Call s_lock() function. tas() as function
InlineTas 0.32 usec Use function inlining. Patch to follow.

Section 3.1 Test Cases and Raw Timing Results.

Case Tag Per Iteration Comments
============= ============= ============================================
TZero: 0.14 usec compare lock to 0, do nothing

#define TAS(lock) (*lock != 0)
#define S_LOCK(lock) TAS(lock)

tryit:
pushl %ebp
movl %esp,%ebp
movl 8(%ebp),%eax
movl 12(%ebp),%edx
movb (%eax),%cl
incl (%edx)
movb $0,(%eax)
movl (%edx),%eax
leave
ret

CPU times: 14.32, 14.32, 14.32

Case Tag Per Iteration Comments
============= ============= ============================================
TZeroNoCall 0.17 usec compare lock to 0, call s_lock if not

#define TAS(lock) (*lock != 0)
#define S_LOCK(lock) if (TAS(lock)) s_lock(lock, __FILE__, __LINE__); else

tryit:
pushl %ebp
movl %esp,%ebp
pushl %esi
pushl %ebx
movl 8(%ebp),%ebx
movl 12(%ebp),%esi
movb (%ebx),%al
testb %al,%al
je .L14
pushl $141
pushl $.LC1
pushl %ebx
call s_lock
.L14:
incl (%esi)
movb $0,(%ebx)
movl (%esi),%eax
leal -8(%ebp),%esp
popl %ebx
popl %esi
leave
ret

CPU times: 17.33, 17.35, 17.31

Case Tag Per Iteration Comments
============= ============= ============================================
TZeroCall 0.30 usec call function that compares lock to 0

#define TAS(lock) tas_test(lock)
#define S_LOCK(lock) if (TAS(lock)) s_lock(lock, __FILE__, __LINE__); else

int tas_test(volatile slock_t *lock)
{
return *lock == 0;
}

tas_test:
pushl %ebp
movl %esp,%ebp
movl 8(%ebp),%eax
movb (%eax),%al
testb %al,%al
setne %al
andl $255,%eax
leave
ret

tryit:
pushl %ebp
movl %esp,%ebp
pushl %esi
pushl %ebx
movl 8(%ebp),%ebx
movl 12(%ebp),%esi
pushl %ebx
call tas_test
addl $4,%esp
testl %eax,%eax
je .L13
pushl $141
pushl $.LC1
pushl %ebx
call s_lock
.L13:
incl (%esi)
movb $0,(%ebx)
movl (%esi),%eax
leal -8(%ebp),%esp
popl %ebx
popl %esi
leave
ret

CPU times: 30.16, 30.15, 30.14

Case Tag Per Iteration Comments
============= ============= ============================================
TasFunction 0.45 usec lock spinlock in separate tas() function

#define TAS(lock) tas(lock)
#define S_LOCK(lock) if (TAS(lock)) s_lock(lock, __FILE__, __LINE__); else

int tas(volatile slock_t *lock)
{
slock_t _res = 1;

__asm__("lock; xchgb %0,%1": "=q"(_res), "=m"(*lock):"0"(0x1));
return _res != 0;
}

tas:
pushl %ebp
movl %esp,%ebp
movl 8(%ebp),%eax
movl $1,%edx
#APP
lock; xchgb %dl,(%eax)
#NO_APP
testb %dl,%dl
setne %al
andl $255,%eax
leave
ret

CPU times: 46.13, 47.48, 45.01, 39.51, 45.79, 45.86

Case Tag Per Iteration Comments
============= ============= ============================================
TasFunction2 0.37 usec. improved separate tas function

#define TAS(lock) tas2(lock)
#define S_LOCK(lock) if (TAS(lock)) s_lock(lock, __FILE__, __LINE__); else

int tas2(volatile slock_t *lock)
{
slock_t _res = 1;

__asm__("lock; xchgb %0,%1": "=q"(_res), "=m"(*lock):"0"(0x1));
return (int) _res;
}

tas2:
pushl %ebp
movl %esp,%ebp
movl 8(%ebp),%edx
movl $1,%eax
#APP
lock; xchgb %al,(%edx)
#NO_APP
andl $255,%eax
leave
ret

CPU times: 37.67, 37.67, 37.68, 37.57, 37.12, 36.91

Case Tag Per Iteration Comments
============= ============= ============================================
SlockAsmMacro 0.31 usec Inline xchgb in S_LOCK macro, call s_lock if
needed. Note strange variation in recorded
times. I have no explaination.

#define TAS(lock) tas2(lock)
#define S_LOCK(lock) if (1) { \
slock_t _res = 1; \
__asm__("lock; xchgb %0,%1": "=q"(_res), "=m"(*lock):"0"(0x1)); \
if (_res) \
s_lock(lock, __FILE__, __LINE__); \
} else

tryit:
pushl %ebp
movl %esp,%ebp
pushl %esi
pushl %ebx
movl 8(%ebp),%ebx
movl 12(%ebp),%esi
movl $1,%eax
#APP
lock; xchgb %al,(%ebx)
#NO_APP
testb %al,%al
je .L14
pushl $141
pushl $.LC1
pushl %ebx
call s_lock
.L14:
incl (%esi)
movb $0,(%ebx)
movl (%esi),%eax
leal -8(%ebp),%esp
popl %ebx
popl %esi
leave
ret

CPU times: 40.53, 30.14, 30.13, 40.44, 30.12, 40.50, 28.65, 28.63, 28.62

Case Tag Per Iteration Comments
============= ============= ============================================
Original 0.31 usec The original S_LOCK from 6.3.2

#define S_LOCK(lock) do { \
slock_t _res = 1; \
do { \
__asm__("lock; xchgb %0,%1": "=q"(_res), "=m"(*lock):"0"(0x1)); \
} while (_res !=0); \
} while (0)

tryit:
pushl %ebp
movl %esp,%ebp
movl 8(%ebp),%edx
movl 12(%ebp),%ecx
.align 4
.align 4
.L15:
movl $1,%eax
#APP
lock; xchgb %al,(%edx)
#NO_APP
testb %al,%al
jne .L15
incl (%ecx)
movb $0,(%edx)
movl (%ecx),%eax
leave
ret

CPU times: 28.55, 33.31, 31.40

Case Tag Per Iteration Comments
============= ============= ============================================
MacroSLOCK 0.45 usec Current in CVS as of 5/30/98. Tends to bloat.

#define TAS(lock) tas(lock)
#define S_LOCK(lock) if (1) { \
int spins = 0; \
while (TAS(lock)) { \
struct timeval delay; \
delay.tv_sec = 0; \
delay.tv_usec = s_spincycle[spins++ % S_NSPINCYCLE]; \
(void) select(0, NULL, NULL, NULL, &delay); \
if (spins > S_MAX_BUSY) { \
/* It's been well over a minute... */ \
s_lock_stuck(lock, __FILE__, __LINE__); \
} \
} \
} else

tryit:
pushl %ebp
movl %esp,%ebp
subl $8,%esp
pushl %edi
pushl %esi
pushl %ebx
movl 8(%ebp),%esi
movl 12(%ebp),%edi
xorl %ebx,%ebx
.align 4
.L13:
pushl %esi
call tas
addl $4,%esp
testl %eax,%eax
je .L18
movl $0,-8(%ebp)
movl %ebx,%edx
movl %ebx,%eax
incl %ebx
testl %edx,%edx
jge .L16
leal 15(%edx),%eax
.L16:
andb $240,%al
subl %eax,%edx
movl %edx,%eax
movl s_spincycle(,%eax,4),%eax
movl %eax,-4(%ebp)
leal -8(%ebp),%eax
pushl %eax
pushl $0
pushl $0
pushl $0
pushl $0
call select
addl $20,%esp
cmpl $16000,%ebx
jle .L13
pushl $141
pushl $.LC1
pushl %esi
call s_lock_stuck
addl $12,%esp
jmp .L13
.align 4
.L18:
incl (%edi)
movb $0,(%esi)
movl (%edi),%eax
leal -20(%ebp),%esp
popl %ebx
popl %esi
popl %edi
leave
ret

CPU times: 49.81, 49.43, 40.68, 49.51, 40.68, 40.69

Case Tag Per Iteration Comments
============= ============= ============================================
AllFunction 0.51 usec Call s_lock() function. tas() as function

#define TAS(lock) tas2(lock)
#define S_LOCK(lock) s_lock(lock, __FILE__, __LINE__)

void s_lock(volatile slock_t *lock, char *file, int line)
{
int spins = 0;

while (TAS(lock))
{
struct timeval delay;

delay.tv_sec = 0;
delay.tv_usec = s_spincycle[spins++ % S_NSPINCYCLE];
(void) select(0, NULL, NULL, NULL, &delay);
if (spins > S_MAX_BUSY)
{
/* It's been well over a minute... */
s_lock_stuck(lock, file, line);
}
}
}

s_lock:
pushl %ebp
movl %esp,%ebp
subl $8,%esp
pushl %edi
pushl %esi
pushl %ebx
movl 8(%ebp),%esi
movl 16(%ebp),%edi
xorl %ebx,%ebx
.align 4
.L5:
pushl %esi
call tas2
addl $4,%esp
testl %eax,%eax
je .L6
movl $0,-8(%ebp)
movl %ebx,%edx
movl %ebx,%eax
incl %ebx
testl %edx,%edx
jge .L8
leal 15(%edx),%eax
.L8:
andb $240,%al
subl %eax,%edx
movl %edx,%eax
movl s_spincycle(,%eax,4),%eax
movl %eax,-4(%ebp)
leal -8(%ebp),%eax
pushl %eax
pushl $0
pushl $0
pushl $0
pushl $0
call select
addl $20,%esp
cmpl $16000,%ebx
jle .L5
pushl %edi
pushl 12(%ebp)
pushl %esi
call s_lock_stuck
addl $12,%esp
jmp .L5
.align 4
.L6:
leal -20(%ebp),%esp
popl %ebx
popl %esi
popl %edi
leave
ret

tryit:
pushl %ebp
movl %esp,%ebp
pushl %esi
pushl %ebx
movl 8(%ebp),%esi
movl 12(%ebp),%ebx
pushl $141
pushl $.LC1
pushl %esi
call s_lock
incl (%ebx)
movb $0,(%esi)
movl (%ebx),%eax
leal -8(%ebp),%esp
popl %ebx
popl %esi
leave
ret

CPU times: 51.23, 51.23, 51.23

Case Tag Per Iteration Comments
============= ============= ============================================
InlineTas 0.32 Use function inlining. Patch to follow.

#define TAS(lock) tas_i(lock)
#define S_LOCK(lock) if (TAS(lock)) s_lock(lock, __FILE__, __LINE__); else

static __inline__ int tas_i(volatile slock_t *lock)
{
slock_t _res = 1;

__asm__("lock; xchgb %0,%1": "=q"(_res), "=m"(*lock): "0"(_res));
return (int) _res;
}

tryit:
pushl %ebp
movl %esp,%ebp
pushl %esi
pushl %ebx
movl 8(%ebp),%ebx
movl 12(%ebp),%esi
movb $1,%al
#APP
lock; xchgb %al,(%ebx)
#NO_APP
testb %al,%al
je .L16
pushl $156
pushl $.LC1
pushl %ebx
call s_lock
.L16:
incl (%esi)
movb $0,(%ebx)
movl (%esi),%eax
leal -8(%ebp),%esp
popl %ebx
popl %esi
leave
ret

CPU times: 39.34, 29.09, 28.65, 40.34, 28.64, 28.64

----------------------------------------------------------------------------
Section 4.0 Test Suite

begin 644 s_lock_test.tar.gz
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end

-dg

David Gould dg@illustra.com 510.628.3783 or 510.305.9468
Informix Software (No, really) 300 Lakeside Drive Oakland, CA 94612
"Don't worry about people stealing your ideas. If your ideas are any
good, you'll have to ram them down people's throats." -- Howard Aiken