MemCached::increment() is not atomic? - php

Question

My understanding is that Memcached::increment is atomic.
I have this code:
include('../clibootstrap.php');
$key = 'ad_1';
$mc = $app['memcache'];
$mc->setOption(Memcached::OPT_BINARY_PROTOCOL,true);
usleep(10);
$mc->increment($key, 1, 0);
die('OK');
$mc is an instance of \MemCached
Now I try to benchmark it using Apache Bench:
# ab -n2000 -c100 http://somehost.com/foo.php
Benchmarking somehost.com (be patient)
Completed 200 requests
Completed 400 requests
Completed 600 requests
Completed 800 requests
Completed 1000 requests
Completed 1200 requests
Completed 1400 requests
Completed 1600 requests
Completed 1800 requests
Completed 2000 requests
Finished 2000 requests
Server Software: Apache/2.2.22
Server Hostname: somehost.com
Server Port: 80
Document Path: /foo.php
Document Length: 2 bytes
Concurrency Level: 100
Time taken for tests: 4.821 seconds
Complete requests: 2000
Failed requests: 0
Write errors: 0
Total transferred: 352000 bytes
HTML transferred: 4000 bytes
Requests per second: 414.82 [#/sec] (mean)
Time per request: 241.067 [ms] (mean)
Time per request: 2.411 [ms] (mean, across all concurrent requests)
Transfer rate: 71.30 [Kbytes/sec] received
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 1.0 0 5
Processing: 33 237 34.5 237 323
Waiting: 33 237 34.5 237 323
Total: 38 237 34.0 237 323
Percentage of the requests served within a certain time (ms)
50% 237
66% 255
75% 263
80% 266
90% 274
95% 279
98% 287
99% 294
100% 323 (longest request)
Now I expect the value of the 'ad_1' key to be exactly 2000, so let's check with telnet:
# telnet localhost 11211
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
get ad_1
VALUE ad_1 0 4
1997
END
version
VERSION 1.4.13
stats
STAT pid 5527
STAT uptime 248
STAT time 1414164851
STAT version 1.4.13
STAT libevent 2.0.16-stable
STAT pointer_size 64
STAT rusage_user 0.092005
STAT rusage_system 0.268016
STAT curr_connections 5
STAT total_connections 2006
STAT connection_structures 23
STAT reserved_fds 20
STAT cmd_get 1
STAT cmd_set 0
STAT cmd_flush 0
STAT cmd_touch 0
STAT get_hits 1
STAT get_misses 0
STAT delete_misses 0
STAT delete_hits 0
STAT incr_misses 0
STAT incr_hits 1997
STAT decr_misses 0
STAT decr_hits 0
STAT cas_misses 0
STAT cas_hits 0
STAT cas_badval 0
STAT touch_hits 0
STAT touch_misses 0
STAT auth_cmds 0
STAT auth_errors 0
STAT bytes_read 144026
STAT bytes_written 112049
STAT limit_maxbytes 67108864
STAT accepting_conns 1
STAT listen_disabled_num 0
STAT threads 4
STAT conn_yields 0
STAT hash_power_level 16
STAT hash_bytes 524288
STAT hash_is_expanding 0
STAT expired_unfetched 0
STAT evicted_unfetched 0
STAT bytes 73
STAT curr_items 1
STAT total_items 1998
STAT evictions 0
STAT reclaimed 0
END
I am using PHP-memcached version 2.2
Any ideas as to why the ad_1 value is not 2000 ?
And how can I make sure that MemCached::increment() becomes atomic?
Also, in the line VALUE ad_1 0 4, what does the 4 mean ?

Answer 1

I used a semaphore, changing my code to
include('../clibootstrap.php');
$key = 'ad_1';
$mc = $app['memcache'];
$mc->setOption(Memcached::OPT_BINARY_PROTOCOL,true);
$sem = sem_get(1234, 1);
if (sem_acquire($sem)) {
$mc->increment($key, 1, 1);
sem_release($sem);
}
usleep(10);
die('OK');
and now it works.