mpm configuration for httpd - linux

Question

I run a website with 5 httpd servers(Centos 7) in EC2, type is m3.2xlarge.
The servers are configured with load balancer.
Gradually the server memory keeps going higher in all the instances.
For example:
Memory usage in few seconds after restarting the httpd service:
[centos#ip-10-0-1-77 ~]$ while sleep 1; do free -m; done
total used free shared buff/cache available
Mem: 29741 2700 26732 36 307 26728
Swap: 0 0 0
total used free shared buff/cache available
Mem: 29741 2781 26651 36 307 26647
Swap: 0 0 0
total used free shared buff/cache available
Mem: 29741 2820 26613 36 307 26609
Swap: 0 0 0
[centos#ip-10-0-1-77 ~]$
.
.
.
This is what i see after an hour:
[centos#ip-10-0-1-77 ~]$ free -m
total used free shared buff/cache available
Mem: 29741 29092 363 41 284 346
Swap: 0 0 0
Like above it goes and consumes all the memory(30GB) within an hour.
To avoid this I started using worker mpm configuration.
The following configuration is what I have added at the bottom of /etc/httpd/httpd.conf.
<IfModule mpm_worker_module>
MaxRequestWorkers 2500
MaxSpareThreads 250
MinSpareThreads 75
ServerLimit 100
StartServers 3
ThreadsPerChild 25
</IfModule>
Can Someone help and suggest me the right configuration to utilize the RAM memory properly in all the instances?

Answer 1

A standard Apache process takes up about 12 MB of RAM. If you have 30 GB reserved for Apache you will never reach that with a serverlimit of 100 (=100*12MB=1200MB=1,2GB). So I assume that Apache isn't taking up all that memory.
Is there an application that is involved or a DB? Those can take up larger amounts of RAM.
For your servertuning.conf (or httpd.conf since you put it there):
<IfModule mpm_worker_module>
#max amount of requests one worker handles before it's forced to close, 2,5k seems almost a little low
MaxRequestsperChild 2500
#maximum number of worker threads which are kept spare
#250 seems quite high, but really depends on the traffic you are experiencing, we normally don't use more than 50-75
MaxSpareThreads 250
#minimum number of worker threads which are kept spare
#really high, only useful if you often experience higher bursts of traffic
#otherwise you should be fine with 15-30, maybe 50 if you experience higher fluctuation -> bigger bursts of requests
MinSpareThreads 75
#upper limit on the configurable number of threads per child process
#you have to increase this if you want more than 64 ThreadsPerChild
ThreadLimit 64
#maximum number of simultaneous client connections
#that is really low! --> increase that, definitely! We run on 1000, so about 12GB max
MaxClients 100
#initial number of server processes to start
#3 is really low, if you expected your server to be flodded with requests the second you start it
#maybe turn it up a little to around 20 or even 50 if you receive lots of traffic right after a restart
StartServers 3
#number of worker threads created by each child proces
#25 threads per worker is not tooo much, but at some point the administration of xx threads gets more expensive than creating new ones
#would suggest to leave it at 25 or turn it up to around 40
ThreadsPerChild 25
</IfModule>
Notice that I changed ServerLimit to MaxClients and MaxRequestWorkers to MaxRequestsPerChild, because as far as I know those are the terms used in mpm-worker.
Additionally you can change following variables:
#KeepAlive: Whether or not to allow persistent connections (more than
#one request per connection). Set to "Off" to deactivate.
#if it's on, leave it there
KeepAlive On
#MaxKeepAliveRequests: The maximum number of requests to allow
#during a persistent connection. Set to 0 to allow an unlimited amount.
#We recommend you leave this number high, for maximum performance.
#default=100, but you can turn that up if your sites contain a lot of item (img, css, ...)
#we are using about 20*<average object-count per site> = 600
MaxKeepAliveRequests 600
#KeepAliveTimeout: Number of seconds to wait for the next request from the
#same client on the same connection.
#would recommend to decrease that, otherwise you could become a victim of slow-dos attacks
#default is 15, we are running just fine on 5
KeepAliveTimeout 5
To further prevent slow-dos or piling up of open sessions, you can use mod_reqtimeout:
<IfModule mod_reqtimeout.c>
# allow 10s timeout for the headers and allow 1s more until 20s upon receipt of 1000 bytes.
# almost the same with the body, except that it is tricky to
# limit the request timeout within the body at all - it may take time to generate the body.
# below are the default values
#RequestReadTimeout header=10-20,MinRate=1000 body=20,MinRate=1000
# and this is how I'd set them with today's internet speed
# deduct the according numbers from explanation above...
RequestReadTimeout header=2-5,MinRate=100000 body=5-10,MinRate=1000000
</IfModule>
If that's not enough to help your RAM-issues (if they are really caused by Apache), use the tools of your server's OS accordingly to find out what is taking up all the RAM --> TOOLS