dormando edited this page May 28, 2016 · 1 revision


Troubleshooting Timeouts

Client complaining about "timeout errors", but not sure how to track it down? Here's a simple utility for examining your situation.

First, check listen_disabled_num

Before you go ahead with troubleshooting, you'll want to telnet to your memcached instance and run stats, then look for "listen_disabled_num". This is a poorly named counter which describes how many times you've reached maxconns. Each time memcached hits maxconns it will delay new connections, which means you'll possibly get timeouts.

Also, disable or tune any firewalls you may have in the way.

Then, carefully check the usual suspects

Is the machine in swap? You will see random lag bubbles if your OS is swapping memcached to disk periodically.

Is the machine overloaded? 0% CPU idle with a load of 400 and memcached probably isn't getting enough CPU time. You can try nice or renice, or just run less on the machine. If you're severely overloaded on CPU, you might notice the mc_conn_tester below reporting very high wait times for set commands.

Is the memcached server 32bit? 32bit hosts have less memory available to the kernel for TCP sockets and friends. We've observed some odd behavior under large numbers of open sockets and high load with 32bit systems. Strongly consider going 64bit, as it may help some hard to trace problems go away, including segfaults due to the 2/4g memory limit.

Next, mc_conn_tester.pl

Fetch this:


$ ./mc_conn_tester.pl -s memcached-host -p 11211 -c 1000 --timeout 1
Averages: (conn: 0.00081381) (set: 0.00001603) (get: 0.00040122)
$ ./mc_conn_tester.pl --help
    mc_conn_tester.pl [options]

    -s --server hostname
            Connect to an alternate hostname.

This is a minimal utility for testing a quick routine with a memcached instance. It will connect, attempt a couple sets, attempt a few gets, then loop and repeat.

The utility does not use any memcached client and instead does minimal, raw commands with the ASCII protocol. Thus helping to rule out client bugs.

If it reaches a timeout, you can see how far along in the cycle it was:

Fail: (timeout: 1) (elapsed: 1.00427794) (conn: 0.00000000) (set: 0.00000000) (get: 0.00000000)
Fail: (timeout: 1) (elapsed: 1.00133896) (conn: 0.00000000) (set: 0.00000000) (get: 0.00000000)
Fail: (timeout: 1) (elapsed: 1.00135303) (conn: 0.00000000) (set: 0.00000000) (get: 0.00000000)
Fail: (timeout: 1) (elapsed: 1.00145602) (conn: 0.00000000) (set: 0.00000000) (get: 0.00000000)

In the above line, it has a total elapsed time of the test, and then the times at which each sub-test succeeded. In the above scanario it wasn't able to connect to memcached, so all tests failed.

Fail: (timeout: 1) (elapsed: 0.00121498) (conn: 0.00114512) (set: 1.00002694) (get: 0.00000000)
Fail: (timeout: 1) (elapsed: 0.00368810) (conn: 0.00360799) (set: 1.00003314) (get: 0.00000000)
Fail: (timeout: 1) (elapsed: 0.00128603) (conn: 0.00117397) (set: 1.00004005) (get: 0.00000000)
Fail: (timeout: 1) (elapsed: 0.00115108) (conn: 0.00108099) (set: 1.00002789) (get: 0.00000000)

In this case, it failed waiting for "get" to complete.

If you want to log all of the tests mc_conn_tester.pl runs, open the file and change the line:

my $debug = 0;


my $debug = 1;

You will then see normal lines begin with loop: and failed tests will start with Fail: as usual.

You're probably dropping packets.

In most cases, where listen_disabled_num doesn't apply, you're likely dropping packets for some reason. Either a firewall is in the way and has run out of stateful tracking slots, or your network card or switch is dropping packets.

You'll most likely see this manifest as:

Fail: (timeout: 1) (elapsed: 1.00145602) (conn: 0.00000000) (set: 0.00000000) (get: 0.00000000)

... where conn: and the rest are all zero. So the test was not able to connect to memcached.

On most systems SYN retries are 3 seconds, which is awfully long. Losing a single SYN packet will certainly mean a timeout. This is easily proven:

$ ./mc_conn_tester.pl -s memcached-host -c 5000 --timeout 1 > log_one_second
&& ./mc_conn_tester.pl -s memcached-host -c 5000 --timeout 4 > log_three_seconds
&& ./mc_conn_tester.pl -s memcached-host -c 5000 --timeout 8 > log_eight_seconds

... Run 5000 tests each round (you can adjust this if you wish). The first one having a timeout of 1s, which is often the client default. Then next with 4s, which would allow for one SYN packet to be lost but still pass the test. Then finally 8s, which allows two SYN packets to be lost in a row and yet still succeed.

If you see the number of Fail: lines in each log file decrease, then your network is likely dropping SYN packets.

Fixing that, however, is beyond the scope of this document.

TIME_WAIT buckets or local port exhaustion

If mc_conn_tester.pl is seeing connection timeouts (conn: is 0), you may be running out of local ports, firewall states, or TIME_WAIT buckets. This can happen if you are opening and closing connections quicker than the sockets can die off.

Use netstat to see how many you have open, and if the number is high enough that it may be problematic. netstat -n | grep -c TIME_WAIT.

Details of how to tune these variables are outside the scope of this document, but google for "Linux TCP network tuning TIME_WAIT" (or whatever OS you have) will usually give you good results. Look for the variables below and understand their meaning before tuning.

net.ipv4.ip_local_port_range = 16384 65534
net.ipv4.tcp_max_tw_buckets = 262144
net.ipv4.tcp_fin_timeout = 30
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_tw_recycle = 1

Also read up on iptables and look up information on managing conntrack states or conntrack buckets. If you find some links you love e-mail us and we'll link them here.

But your utility never fails!

Odds are good your client has a bug :( Try reaching out to the client author for help.