Paolo Bonzini writes:
> On 25/10/2016 17:49, Pranith Kumar wrote:
>> But we are taking the seqlock of only the head bucket, while the
>> readers are reading hashes/pointers of the chained buckets.
>
> No, we aren't. See qht_lookup__slowpath.
I don't see it. The reader is taking the head bucket look in
qht_lookup__slowpath() and then iterating over the chained buckets in
qht_do_lookup(). The writer is doing the same. It is taking the head bucket
lock in qht_insert__locked().
I've written a patch (see below) to take the per-bucket sequence locks.
>
> This patch:
>
> throughput base patch %change
> update
> 0 8.07 13.33 +65%
> 10 7.10 8.90 +25%
> 20 6.34 7.02 +10%
> 30 5.48 6.11 +9.6%
> 40 4.90 5.46 +11.42%
>
>
> Just doubling the cachesize:
>
> throughput base patch %change
> update
> 0 8.07 4.47 -45% ?!?
> 10 7.10 9.82 +38%
> 20 6.34 8.13 +28%
> 30 5.48 7.13 +30%
> 40 5.90 6.45 +30%
>
> It seems to me that your machine has 128-byte cachelines.
>
Nope. It is just the regular 64 byte cache line.
$ getconf LEVEL1_DCACHE_LINESIZE
64
(The machine model is Xeon CPU E5-2620).
Take the per-bucket sequence locks instead of the head bucket lock.
Signed-off-by: Pranith Kumar <bobby.pr...@gmail.com>
---
util/qht.c | 36 ++++++++++++++++++------------------
1 file changed, 18 insertions(+), 18 deletions(-)
diff --git a/util/qht.c b/util/qht.c
index 4d82609..cfce5fc 100644
--- a/util/qht.c
+++ b/util/qht.c
@@ -374,19 +374,19 @@ static void qht_bucket_reset__locked(struct qht_bucket
*head)
struct qht_bucket *b = head;
int i;
- seqlock_write_begin(&head->sequence);
do {
+ seqlock_write_begin(&b->sequence);
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (b->pointers[i] == NULL) {
- goto done;
+ seqlock_write_end(&b->sequence);
+ return;
}
atomic_set(&b->hashes[i], 0);
atomic_set(&b->pointers[i], NULL);
}
+ seqlock_write_end(&b->sequence);
b = b->next;
} while (b);
- done:
- seqlock_write_end(&head->sequence);
}
/* call with all bucket locks held */
@@ -446,6 +446,8 @@ void *qht_do_lookup(struct qht_bucket *head,
qht_lookup_func_t func,
int i;
do {
+ void *q = NULL;
+ unsigned int version = seqlock_read_begin(&b->sequence);
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (atomic_read(&b->hashes[i]) == hash) {
/* The pointer is dereferenced before seqlock_read_retry,
@@ -455,11 +457,16 @@ void *qht_do_lookup(struct qht_bucket *head,
qht_lookup_func_t func,
void *p = atomic_rcu_read(&b->pointers[i]);
if (likely(p) && likely(func(p, userp))) {
- return p;
+ q = p;
+ break;
}
}
}
- b = atomic_rcu_read(&b->next);
+ if (!q) {
+ b = atomic_rcu_read(&b->next);
+ } else if (!seqlock_read_retry(&b->sequence, version)) {
+ return q;
+ }
} while (b);
return NULL;
@@ -469,14 +476,7 @@ static __attribute__((noinline))
void *qht_lookup__slowpath(struct qht_bucket *b, qht_lookup_func_t func,
const void *userp, uint32_t hash)
{
- unsigned int version;
- void *ret;
-
- do {
- version = seqlock_read_begin(&b->sequence);
- ret = qht_do_lookup(b, func, userp, hash);
- } while (seqlock_read_retry(&b->sequence, version));
- return ret;
+ return qht_do_lookup(b, func, userp, hash);
}
void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
@@ -537,14 +537,14 @@ static bool qht_insert__locked(struct qht *ht, struct
qht_map *map,
found:
/* found an empty key: acquire the seqlock and write */
- seqlock_write_begin(&head->sequence);
+ seqlock_write_begin(&b->sequence);
if (new) {
atomic_rcu_set(&prev->next, b);
}
/* smp_wmb() implicit in seqlock_write_begin. */
atomic_set(&b->hashes[i], hash);
atomic_set(&b->pointers[i], p);
- seqlock_write_end(&head->sequence);
+ seqlock_write_end(&b->sequence);
return true;
}
@@ -665,9 +665,9 @@ bool qht_remove__locked(struct qht_map *map, struct
qht_bucket *head,
}
if (q == p) {
qht_debug_assert(b->hashes[i] == hash);
- seqlock_write_begin(&head->sequence);
+ seqlock_write_begin(&b->sequence);
qht_bucket_remove_entry(b, i);
- seqlock_write_end(&head->sequence);
+ seqlock_write_end(&b->sequence);
return true;
}
}
--
2.10.1
