> On the other hand I think we will have to come up with something
> more clever than what I do now because for some huge machines with
> nr_cpu_ids == 256, the error of the counter is 256*9*8 = 18432 so that's
> already unacceptable given the amounts we want to check (like 1536) -
> already for nr_cpu_ids == 32, the error is the same as the difference we
> want to check. I think we'll have to come up with some scheme whose error
> is not dependent on the number of cpus or if it is dependent, it's only a
> weak dependency (like a logarithm or so).
> Or we could rely on the fact that IO completions for a bdi won't happen on
> all CPUs and thus the error would be much more bounded. But I'm not sure
> how much that is true or not.
Yes the per CPU counter seems tricky. How about plain atomic operations?
This test shows that atomic_dec_and_test() is about 4.5 times slower
than plain i-- in a 4-core CPU. Not bad.
Note that
1) we can avoid the atomic operations when there are no active waiters
2) most writeback will be submitted by one per-bdi-flusher, so no worry
of cache bouncing (this also means the per CPU counter error is
normally bounded by the batch size)
3) the cost of atomic inc/dec will be weakly related to core numbers
but never socket numbers (based on 2), so won't scale too bad
Thanks,
Fengguang
---
$ perf stat ./atomic
Performance counter stats for './atomic':
903.875304 task-clock-msecs # 0.998 CPUs
76 context-switches # 0.000 M/sec
0 CPU-migrations # 0.000 M/sec
98 page-faults # 0.000 M/sec
3011186459 cycles # 3331.418 M/sec
1608926490 instructions # 0.534 IPC
301481656 branches # 333.543 M/sec
94932 branch-misses # 0.031 %
88687 cache-references # 0.098 M/sec
1286 cache-misses # 0.001 M/sec
0.905576197 seconds time elapsed
$ perf stat ./non-atomic
Performance counter stats for './non-atomic':
215.315814 task-clock-msecs # 0.996 CPUs
18 context-switches # 0.000 M/sec
0 CPU-migrations # 0.000 M/sec
99 page-faults # 0.000 M/sec
704358635 cycles # 3271.281 M/sec
303445790 instructions # 0.431 IPC
100574889 branches # 467.104 M/sec
39323 branch-misses # 0.039 %
36064 cache-references # 0.167 M/sec
850 cache-misses # 0.004 M/sec
0.216175521 seconds time elapsed
--------------------------------------------------------------------------------
$ cat atomic.c
#include <stdio.h>
typedef struct {
int counter;
} atomic_t;
static inline int atomic_dec_and_test(atomic_t *v)
{
unsigned char c;
asm volatile("lock; decl %0; sete %1"
: "+m" (v->counter), "=qm" (c)
: : "memory");
return c != 0;
}
int main(void)
{
atomic_t i;
i.counter = 100000000;
for (; !atomic_dec_and_test(&i);)
;
return 0;
}
--------------------------------------------------------------------------------
$ cat non-atomic.c
#include <stdio.h>
int main(void)
{
int i;
for (i = 100000000; i; i--)
;
return 0;
}
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