OS Level Tuning

To get the most out of Sync Gateway, it may be necessary to tune a few parameters of the OS.

Tuning the max no. of file descriptors

Raising the maximum number of file descriptors available to Sync Gateway is important because it directly affects the maximum number of sockets the Sync Gateway can have open, and therefore the maximum number of endpoints that the Sync Gateway can support.

Linux Instructions (CentOS)

The following instructions are geared towards CentOS.

Global limits

Increase the max number of file descriptors available to all processes. To specify the number of system wide file descriptors allowed, open up the /etc/sysctl.conf file and add the following line.

fs.file-max = 500000

Apply the changes and persist them (this will last across reboots) by running the following command.

$ sysctl -p

Sync Gateway Config Limits

The maximum number of open files descriptors should also be configured in the Sync Gateway configuration file. Refer to the server section of the configuration guide and to the example below.

{
  "maxFileDescriptors": 250000,
  "databases": {
    "todo": { ... }
  }
}

Ulimits systemd config

The /usr/lib/systemd/system/sync_gateway.service has a hardcoded limit specified by LimitNOFILE=65535. To increase that, edit the /sync_gateway.service file to your desired value and restart the service.

Ulimits CLI

If you are running Sync Gateway outside of systemd, use the following instructions. If you are using systemd, you can skip this section.

Increase the ulimit setting for max number of file descriptors available to a single process. For example, setting it to 250K will allow the Sync Gateway to have 250K connections open at any given time, and leave 250K remaining file descriptors available for the rest of the processes on the machine. These settings are just an example, you will probably want to tune them for your own particular use case.

$ ulimit -n 250000

In order to persist the ulimit change across reboots, add the following lines to /etc/security/limits.conf.

* soft nofile 250000
* hard nofile 250000

Verify your changes by running the following commands.

$ cat /proc/sys/fs/file-max
$ ulimit -n

The value of both commands above should be 250000.

References

Tuning the TCP Keepalive parameters

If you have already raised the maximum number of file descriptors available to Sync Gateway, but you are still seeing "too many open files" errors, you may need to tune the TCP Keepalive parameters.

Understanding the problem

Mobile endpoints tend to abruptly disconnect from the network without closing their side of the connection, as described in Section 2.3. (Checking for dead peers) of the TCP-Keepalive-HOWTO.

By default, these connections will hang around for approximately 7200 seconds (2 hours) before they are detected to be dead and cleaned up by the tcp/ip stack of the Sync Gateway process. If enough of these connections accumulate, you can end up seeing "too many open files" errors on Sync Gateway.

If you are seeing "too many open files" errors, you can count the number of established connections coming into your sync gateway with the following command:

$ lsof -p <sync_gw_pid> | grep -i established | wc -l

If the value returned is near your max file descriptor limit, then you can either try increasing the max file descriptor limit even higher, or tuning the TCP Keepalive parameters to reduce the amount of time that dead peers will cause a socket to be held open on their behalf.

Linux Instructions (CentOS)

Tuning the TCP Keepalive settings is not without its downsides — it will increase the amount of overall network traffic on your system, because the tcp/ip stack will be sending more frequent Keepalive packets in order to detect dead peers faster.

The following settings will reduce the amount of time that dead peer connections hang around from approximately 2 hours down to approximately 30 minutes. Add the following lines to your /etc/sysctl.conf file:

net.ipv4.tcp_keepalive_time = 600
net.ipv4.tcp_keepalive_intvl = 60
net.ipv4.tcp_keepalive_probes = 20

This translates to:

  1. The keepalive routines wait initially for 10 minutes (600 secs) before sending the first keepalive probe.

  2. Resend the probe every minute (60 seconds).

  3. If no ACK response is received for 20 consecutive times, the connection is marked as broken.

To reduce the amount of time even further, you can reduce the tcp_retries2 value. Add the following line to your /etc/sysctl.conf file:

net.ipv4.tcp_retries2 = 8

To activate the changes and persist them across reboots, run:

$ sysctl -p

See Using TCP keepalive under Linux for more details on setting these parameters.

References