Avoid multiple dns lookups for the same query
If two or more rapid dns requests for the same server are done
from different threads it turns into separate dns reques, if
the response of the request isn't found in the cache.
This patch avoid multiple request for the same server by
letting subsequents request wait until the first request
has finished.
Change-Id: Ic72ea0e7d3964a4164eddf866feb4357ec4dfe54
diff --git a/libc/netbsd/resolv/res_cache.c b/libc/netbsd/resolv/res_cache.c
index 284ca2f..2dcb164 100644
--- a/libc/netbsd/resolv/res_cache.c
+++ b/libc/netbsd/resolv/res_cache.c
@@ -1159,6 +1159,15 @@
* inlined in the Entry structure.
*/
+/* Maximum time for a thread to wait for an pending request */
+#define PENDING_REQUEST_TIMEOUT 20;
+
+typedef struct pending_req_info {
+ unsigned int hash;
+ pthread_cond_t cond;
+ struct pending_req_info* next;
+} PendingReqInfo;
+
typedef struct resolv_cache {
int max_entries;
int num_entries;
@@ -1167,6 +1176,7 @@
unsigned generation;
int last_id;
Entry* entries;
+ PendingReqInfo pending_requests;
} Cache;
typedef struct resolv_cache_info {
@@ -1181,6 +1191,107 @@
#define HTABLE_VALID(x) ((x) != NULL && (x) != HTABLE_DELETED)
static void
+_cache_flush_pending_requests_locked( struct resolv_cache* cache )
+{
+ struct pending_req_info *ri, *tmp;
+ if (cache) {
+ ri = cache->pending_requests.next;
+
+ while (ri) {
+ tmp = ri;
+ ri = ri->next;
+ pthread_cond_broadcast(&tmp->cond);
+
+ pthread_cond_destroy(&tmp->cond);
+ free(tmp);
+ }
+
+ cache->pending_requests.next = NULL;
+ }
+}
+
+/* return 0 if no pending request is found matching the key
+ * if a matching request is found the calling thread will wait
+ * and return 1 when released */
+static int
+_cache_check_pending_request_locked( struct resolv_cache* cache, Entry* key )
+{
+ struct pending_req_info *ri, *prev;
+ int exist = 0;
+
+ if (cache && key) {
+ ri = cache->pending_requests.next;
+ prev = &cache->pending_requests;
+ while (ri) {
+ if (ri->hash == key->hash) {
+ exist = 1;
+ break;
+ }
+ prev = ri;
+ ri = ri->next;
+ }
+
+ if (!exist) {
+ ri = calloc(1, sizeof(struct pending_req_info));
+ if (ri) {
+ ri->hash = key->hash;
+ pthread_cond_init(&ri->cond, NULL);
+ prev->next = ri;
+ }
+ } else {
+ struct timespec ts = {0,0};
+ ts.tv_sec = _time_now() + PENDING_REQUEST_TIMEOUT;
+ int rv = pthread_cond_timedwait(&ri->cond, &cache->lock, &ts);
+ }
+ }
+
+ return exist;
+}
+
+/* notify any waiting thread that waiting on a request
+ * matching the key has been added to the cache */
+static void
+_cache_notify_waiting_tid_locked( struct resolv_cache* cache, Entry* key )
+{
+ struct pending_req_info *ri, *prev;
+
+ if (cache && key) {
+ ri = cache->pending_requests.next;
+ prev = &cache->pending_requests;
+ while (ri) {
+ if (ri->hash == key->hash) {
+ pthread_cond_broadcast(&ri->cond);
+ break;
+ }
+ prev = ri;
+ ri = ri->next;
+ }
+
+ // remove item from list and destroy
+ if (ri) {
+ prev->next = ri->next;
+ pthread_cond_destroy(&ri->cond);
+ free(ri);
+ }
+ }
+}
+
+/* notify the cache that the query failed */
+void
+_resolv_cache_query_failed( struct resolv_cache* cache,
+ const void* query,
+ int querylen)
+{
+ Entry key[1];
+
+ if (cache && entry_init_key(key, query, querylen)) {
+ pthread_mutex_lock(&cache->lock);
+ _cache_notify_waiting_tid_locked(cache, key);
+ pthread_mutex_unlock(&cache->lock);
+ }
+}
+
+static void
_cache_flush_locked( Cache* cache )
{
int nn;
@@ -1197,6 +1308,9 @@
}
}
+ // flush pending request
+ _cache_flush_pending_requests_locked(cache);
+
cache->mru_list.mru_next = cache->mru_list.mru_prev = &cache->mru_list;
cache->num_entries = 0;
cache->last_id = 0;
@@ -1470,7 +1584,17 @@
if (e == NULL) {
XLOG( "NOT IN CACHE");
- goto Exit;
+ // calling thread will wait if an outstanding request is found
+ // that matching this query
+ if (!_cache_check_pending_request_locked(cache, key)) {
+ goto Exit;
+ } else {
+ lookup = _cache_lookup_p(cache, key);
+ e = *lookup;
+ if (e == NULL) {
+ goto Exit;
+ }
+ }
}
now = _time_now();
@@ -1573,6 +1697,7 @@
_cache_dump_mru(cache);
#endif
Exit:
+ _cache_notify_waiting_tid_locked(cache, key);
pthread_mutex_unlock( &cache->lock );
}
diff --git a/libc/netbsd/resolv/res_send.c b/libc/netbsd/resolv/res_send.c
index dbad6dd..2f131f1 100644
--- a/libc/netbsd/resolv/res_send.c
+++ b/libc/netbsd/resolv/res_send.c
@@ -646,6 +646,9 @@
errno = terrno;
return (-1);
fail:
+#if USE_RESOLV_CACHE
+ _resolv_cache_query_failed(cache, buf, buflen);
+#endif
res_nclose(statp);
return (-1);
}
diff --git a/libc/private/resolv_cache.h b/libc/private/resolv_cache.h
index 2a54453..1dcc53f 100644
--- a/libc/private/resolv_cache.h
+++ b/libc/private/resolv_cache.h
@@ -95,4 +95,10 @@
const void* answer,
int answerlen );
+/* Notify the cache a request failed */
+extern void
+_resolv_cache_query_failed( struct resolv_cache* cache,
+ const void* query,
+ int querylen);
+
#endif /* _RESOLV_CACHE_H_ */
