Applicable to:
- Plesk for Linux
Symptoms
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Websites that have PHP handler set as FPM application served by Apache or FPM application served by nginx at Domains > example.com > PHP Settings load slowly or are not accessible (permanently or periodically) with a 50x error in a web-browser:
PLESK_INFO: 500 Internal Server Error
OR
PLESK_INFO: 502 Bad Gateway
OR
PLESK_INFO: 503 Service Temporarily Unavailable
OR
PLESK_INFO: 504 Gateway Time-out
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The following message (in bold) appears in the PHP-FPM error logfiles constantly:
# grep -r "server reached max_children setting" /var/log/*php*-fpm* | wc -l
576In some cases, the error message may be as follows:
# grep -r "seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers)" /var/log/*php*-fpm* | wc -l
576-
Error messages above are usually accompanied with this error:
# grep -r "unable to read what child say: Bad file descriptor (9)" /var/log/*php*-fpm* | wc -l
132
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Server performance may decrease due to php-fpm processes that use up to 100% of CPU.
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If nginx is enabled on the server, one of the following error messages may appear in domain's error logfile
/var/www/vhosts/system/example.com/logs/proxy_error_log:CONFIG_TEXT: [error] 11456#0: *118348 connect() to unix:///var/www/vhosts/system/example.com/php-fpm.sock failed (11: Resource temporarily unavailable) while connecting to upstream, client: 203.0.113.2, server: example.com, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:///var/www/vhosts/system/example.com/php-fpm.sock:", host: "example.com"
OR
CONFIG_TEXT: [error] 20058#0: *4725139 upstream prematurely closed connection while reading response header from upstream, client: 203.0.113.2, server: example.com, request: "POST /path/to/some/file HTTP/2.0", upstream:
Cause
New child processes of PHP-FPM master process cannot be spawned due to reached the pm.max_children limit. This may happen due to:
- high website traffic
- unoptimized application code, which may cause long living PHP-FPM processes hitting limit overuse. Website connections are not closed properly and remain hanged, until
pm.max_childrenfor a domain is reached.
Resolution
Adjust the pm.max_children limit for affected domains:
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Connect to the Plesk server via SSH.
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Find affected domains using the commands below:
Note: The number on the left indicates how many times the the
pm.max_childrenlimit has been reached by the website.-
If the message is "server reached max_children setting":
# grep -r "server reached max_children setting" /var/log/*php*-fpm* | cut -d' ' -f5 | tr -d ']' | sort | uniq -c | sort -nr
300 example.com
200 example.net
100 example.org
<...> -
If the message is "pool [example.com] seems busy":
# grep -r "seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers)" /var/log/*php*-fpm* | cut -d' ' -f5 | tr -d ']' | sort | uniq -c | sort -nr
300 example.com
200 example.net
100 example.org
<...>
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Increase the
pm.max_childrenlimit for the listed above domains.Note that the server performance can be affected if
pm.max_childrenis too big. In this example, we are settingpm.max_childrento 20.-
Plesk Onyx 17.5 and newer releases
Increasing pm.max_children for a domain-
In Plesk, go to Domains > example.com > PHP Settings > PHP-FPM Settings.
-
Increase the
pm.max_childrenvalue for a domain and apply the changes.
Increasing pm.max_children for all domains under a service plan-
In Plesk, go to Service Plans > plan_name > PHP Settings tab > PHP-FPM Settings section.
-
Increase the
pm.max_childrenvalue and click Update & Sync.
Note: The changes will not be applied to subscriptions if:
- A subscription is not synced with this service plan (Marked with a blue lock in the Subscriptions menu).
- If the permission Hosting performance settings management is granted in service plan settings at Service Plans > plan_name > Permissions tab.
Click on the picture to enlarge
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Plesk Onyx 17.0 and previous releases
Increasing pm.max_children for one domain-
Create the
php.inifile in domain's system directory/var/www/vhosts/system/example.com/conf/:# touch /var/www/vhosts/system/example.com/conf/php.ini
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Open the
php.inifile in any text editor (for example, vi editor) and add the following records:CONFIG_TEXT: [php-fpm-pool-settings]
pm.max_children = 20 -
Save the changes and close the file.
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Run the following command to apply the changes:
# /usr/local/psa/bin/php_settings -u
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If raising the pm.max_children parameter considerably does not help, consider to:
-
Review website code to understand the reason of hanging connections.
Note: PHP profilers such as xdebug may help to find out the bottleneck at expense of lower performance during the profiling.
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Switch to PHP 7.x, if 5.x is used on the domain:
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In Plesk, go to Domains > example.com > PHP settings.
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Set PHP version option value to PHP 7.x.
Note: Some applications might not be compatible with newer PHP versions. To find out if the application is compatible, refer to its documentation.
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Comments
12 comments
Great help and tutorial ! Thanks a lot
Possible to apply this setting to all domains with single file editing? I have lots of domains to be configured 1 by 1
@Sharul Hafiz You can check the article mentioned in Additional Information section - https://support.plesk.com/hc/en-us/articles/115001201949
Keep in mind that increasing of pm.max_children parameter for all domains can affect the server performance significantly. It is better to filter only affected domains in /var/log/plesk-phpXX-fpm/error.log file
===
grep "server reached max_children setting" /var/log/plesk-phpXX-fpm/error.log
===
and adjust max_children just for them.
@Nikolay What is the limit of pm.max_children? let say if I have 10GB free ram.
@Sharul
The setting is purely depends on amount of traffic for the website. Try to double the default value (from 5 to 10) and monitor the site for several days.
If the same error appears again, analyze the time period between reaching MaxChildren. In case the time like several minutes, double the value again (from 10 to 20), in case the error appears once a day, just add one more child to the configured value (20 to 21).
Typically, 20 is more than enough for medium-loaded websites.
This line
# grep "server reached max_children setting" /var/log/php-fpm/error.log | awk '{print $5}' | sort -n | uniq -cis wrong, as it should be
# grep "server reached max_children setting" /var/log/plesk-phpXX-fpm/error.log | awk '{print $5}' | sort -n | uniq -cMay you insert
in Additional PHP directives field of PHP Settings page for domain?
Hello Marco,
Both files (/var/log/php-fpm/error.log and /var/log/plesk-phpXX-fpm/error.log) exist on the filesystem. The first is used to store errors from system php-fpm, and the second is used when we are speaking about php-fpm which is provided bu Plesk.
Meanwhile, I have edited the article in this way:
# grep "server reached max_children setting" /var/log/*php*-fpm/error.log | awk '{print $5}' | sort -n | uniq -c
As for the second question, Additional PHP directives field is used to insert configuration directives in /var/www/vhosts/system/example.com/conf/php.ini file (configuration file for php itself), however pm.max_children directive should be added to the configuration file of php-fpm pool (/var/www/vhosts/system/example.com/conf/php.ini).
So, this is two different files. And now Plesk could manage only php settings, but not php fpm pool settings.
If you would like Plesk to manage php fpm pool settings, vote for this feature on Plesk Uservoice portal.
@Konstantin
You provided the same path for both php.ini files and said these were two different files.
Hello @Lars,
Thank you for noticing. Indeed, additional directives are stored in/var/www/vhosts/system/example.com/conf/php.ini and PHP-FPM settings are stored in /opt/plesk/php/X.X/etc/php-fpm.d/example.com.conf.
As it is stated in the article, now it is possible to change PHP-FPM settings in Pleks GUI.
I'd like to create a weekly scheduled task (cron job) to check for "server reached max_children setting" in the plesk-php*-fpm error.log, on CentOS 7. It looks like that log is rotated weekly.
Could someone make a suggestion for a good way to schedule a weekly task, which will e-mail the server administrator, weekly, with a list of domains that have reached "max_children"? I suppose I could create something in "/etc/cron.weekly", but how do I make sure the script runs before the weekly log rotation?
Hello @Bob B!
You are right, the log files from the directories /var/log/plesk-phpXX-fpm/ are rotated weekly - in the logrotate configuration files /etc/logrotate.d/plesk-phpXX, options daily, weekly, monthly, or yearly are not specified, and in the main configuration file /etc/logrotate.conf, the option weekly is set.
For the weekly task you would like to create, you can write a corresponding script and put it into the option prerotate/endscript in the logrotate configuration files /etc/logrotate.d/plesk-phpXX. For more details on logrotate configuration options, see the link https://linux.die.net/man/8/logrotate. Please note that the script should be made on your own.
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