Thanks for the details!
I now have a better picture on how it works.
I employed all the recommendations you suggested. I will keep monitoring
the processes.
But how about the graceful restart signaled by Apache? Why it happened?
Thanks again.
On Friday, October 31, 2014 12:29:12 PM UTC+8, Graham Dumpleton wrote:
>
> So lets look at just memory initially.
>
> When we talk about Apache processes, there are actually going to be three
> different classes of processes.
>
> These are:
>
> 1. The Apache parent process.
>
> This runs as root. Because it runs as root and not the Apache user
> (www-data) that the Apache child worker processes run as, then it can be
> seen separately on the NR server monitoring.
>
> There is no memory increase here. It is actually very important that the
> Apache parent process doesn't increase in size over time with restarts as
> when it forks child process they would inherit that increasing memory size.
> One of the reason that using mod_python is a bad idea is that it used to
> initialise the Python interpreter in the Apache parent process and when
> restarted, the Python interpreter would leak memory and so the Apache
> parent process would grow in size on each restart.
>
> Anyway, you are good here and nothing to worry about.
>
> 2. The Apache child worker processes.
>
> These run as the Apache user which on your system is www-data.
>
> The Apache child worker processes are what accepts all inbound HTTP
> requests. Static files will be served up by these processes.
>
> In your case where you are using mod_wsgi daemon processes, the Python web
> application does not run in the Apache child worker processes. The Apache
> child worker process still accept the requests initially though and would
> then proxy the request through to the daemon process running the WSGI
> application.
>
> 3. The mod_wsgi daemon processes.
>
> By default these also run as the Apache user which on your system is
> www-data.
>
> This is where your WSGI application is running.
>
> Now both the Apache child worker processes and the mod_wsgi daemon
> processes are forked children of the Apache parent process. They will
> therefore all show as being the process 'apache2' on your system.
>
> Because both (1) and (2) are running as the Apache user, NR server
> monitoring will not allow you to distinguish between them.
>
> Thus when you get:
>
> you aren't looking at just the Apache child worker processes and are also
> looking at the mod_wsgi daemon processes.
>
> The only way you could use NR server monitoring to tell between the two is
> if you used the 'user' option to WSGIDaemonProcess and ran the mod_wsgi
> daemon processes as a different user again. Then you could distinguish.
>
> Although you can't distinguish using NR server monitoring, you can on
> Linux if you ensure you are naming the processes. That is, when using
> WSGIDaemonProcess directive, supply the display-name=%{GROUP} option. This
> will allow you to see the mod_wsgi daemon processes as distinct if using
> 'ps' or 'htop'.
>
> Lets instead now look at what the mod_wsgi plugin tells you:
>
> Over the 24 hours matching last charts we get:
>
> so nothing conclusive. For a shorter period of one hour we get:
>
> This shows an increase over time, but you have to be a little bit careful
> in interpreting that. If you hover over the memory chart and look at the
> popup you will see:
>
> This shows that the processes actually range in size. There is a range of
> sizes because the process churn chart shows that processes are being
> restarted at different times.
>
> Important though is to look at the max value. This is the largest size of
> any process. If one moves along the chart for that period one sees that the
> maximum is always about 12.9-13.0 MB.
>
> This indicates to me that the Apache child worker process size has reached
> a plateau and that this is the normal behaviour of Apache child process to
> grow over time up to a plateau which I believe I noted previously.
>
> So for the Apache child worker processes this isn't ringing any alarm
> bells for me.
>
> If you have already removed MaxConnectionsPerChild and that maximum
> continues along and not increasing too much more over time, then I wouldn't
> be seeing a need to be setting MaxConnectionsPerChild.
>
> Next chart to look at is the memory chart where monitoring the actual
> Python web application.
>
> Over the last 6 hours this shows:
>
> I am not 100% sure if you can get to this chart on the Lite subscription
> level you are on but I can still see it.
>
> If it is available on Lite subscription, go to the bottom of the overview
> dashboard and select on the chart icon for the server breakout.
>
> So as far as what process is growing in memory size, it is very much the
> mod_wsgi daemon processes.
>
> The problem therefore is going to be an issue with caching in your
> application, construction of object cycles that cannot be broken by the
> garbage collector, or in worst case the garbage collector isn't even
> running due to it dead locking.
>
> As a short term remedy to at least keep the memory growth under control
> for the mod_wsgi daemon processes, you have two choices.
>
> The first is to use the 'inactivity-timeout' option to WSGIDaemonProcess
> directive.
>
> The behaviour of this is slightly different between mod_wsgi 3.X and 4.X.
>
> Under 4.X what will happen when inactivity-timeout is set is that if no
> requests at all are received by a daemon process group, and there are no
> active ones, after the specified period, then the process will be restarted.
>
> You run up to 150 requests per minute across about 7 process instances.
>
>
> In practice this means that any one process is not even handling one
> request per second.
>
> Even so, what it means is that if you set this to be 30 seconds, then a
> process is never going to be idle long enough to be restarted. Can't
> therefore use this.
>
> On mod_wsgi 3.X the difference was that the reading of request content and
> writing of response content was monitored and if there was no activity on
> those after the timeout period, then the process would be restarted. With
> the way it worked, it meant if there was a long running request, or stuck
> requests, they would be interrupted. In mod_wsgi 4.X, you should be using
> request-timeout instead if the intent is to be able to interrupt long
> running or stuck requests.
>
> The second option you can use instead of inactivity-timeout is the
> maximum-requests option to WSGIDaemonProcess.
>
> This also has changed a bit in mod_wsgi 4.X from 3.X.
>
> Under 3.X once that many requests were processed by a process, the process
> would be restarted. In restarting though, it followed the typical shutdown
> sequence for mod_wsgi daemon processes.
>
> That is, no new requests would be accepted on that process. Currently
> running requests would be given a grace period to complete (default 2.5
> seconds). If they did not complete in that time, the process would be more
> brutally shutdown.
>
> The result of this was that long running requests could be interrupted,
> resulting in some of those errors you saw before.
>
> In mod_wsgi 4.X this option has the same default behaviour, but there is a
> new way of having the restart be more graceful.
>
> What you instead use is both maximum-requests and graceful-timeout to
> WSGIDaemonProcess at the same time.
>
> When you also have graceful-timeout option specified, what will happen
> when the maximum requests is reached is that a new initial grace period is
> entered based on what graceful-timeout is set to.
>
> During this initial grace period, new requests will still be accepted, but
> if at any point during that time there are no active requests, the process
> will be restarted immediately.
>
> If for some reason the initial grace period expires, then the normal
> restart starts. That is, stop accepting requests, provide a further 2.5
> seconds grace and then brutally restart.
>
> So this new graceful-timeout option in conjunction with maximum-requests
> means that a process can be restarted but with a better ability to do so
> without interrupting any requests.
>
> In your case because there is less than 1 request per second on average
> across all processes, there should be a lot of opportunity for a restart to
> occur in the gaps in between those infrequent requests.
>
> My recommendations at this point are:
>
> 1. Set MaxConnectionsPerChild to 0 so that Apache child worker processes
> aren't being recycled unnecessarily.
>
> Monitor the mod_wsgi plugin chart 'Average Memory Useds (bytes/process)'
> to ensure that the Apache child worker processes aren't growing in size in
> an unbounded way above the typical max you are seeing now. They should
> simply plateau after a while. Keep an eye on max in the popup rather than
> the average as the latter can be deceiving as Apache child worker processes
> can still be restarted by Apache because of its dynamic scaling feature,
> thereby confusing the results you see.
>
> 2. Add to WSGIDaemonProcess the maximum-requests and graceful-timeout
> option.
>
> As you are running only about 20 requests per minute for process, set
> maximum-requests at about 250 initially, which would be about every 10
> minutes or so at that rate. If the processes are getting recycled too often
> and the startup cost is too much, then increase it. At the same time though
> keep an eye on that memory chart in the server breakout on the main
> dashboard to ensure memory not growing too much before the process gets
> recycled.
>
> At the same time, make sure you set graceful-timeout. A safe figure for
> this initially would be 30. That is the number of seconds to wait to see if
> can restart when no requests, before a more brutal restart.
>
> 3. If you haven't done it already and you are definitely only using
> mod_wsgi daemon mode for running your WSGI application, ensure you are
> setting WSGIRestrictEmbedded to On at global scope in the Apache
> configuration outside of any VirtualHost.
>
> So as an initial remedial action only, to combat memory growth, do that
> for now. This will give you some breathing space so you can work out why
> your web application is growing memory usage continually.
>
> When done and when it has been running for 6-12 hours, come back and we
> can review how the memory growth charts are looking. I can also get to some
> other charts you can't when on Lite subscription, which will show details
> about the mod_wsgi daemon process restarts and we can see how far apart
> they are coming and whether we need to adjust maximum-requests.
>
> As I say though, this is just to get you in a better position while you
> work out your memory leakage problems.
>
> A next step I would like to do is have you install an extension to the NR
> Python agent which allows the activities of the garbage collector to be
> monitored to make sure it is running and whether uncollectable objects are
> accumulating. You would not normally be able to use this with a Lite
> account but I will be able to see the data. Before I can give you that
> extension to try I need to wait on some internal approvals to release it,
> which are already in train due to us needing to provide it to some other
> users who are seeing memory growth issues which we are trying to help them
> with.
>
> For interests sake, the charts it can produce are:
>
> [image: custom-dashboard.jpg]
> Anyway, work slowy through what I have said above and understand it. Get
> those few configuration changes made and then we will look at the next
> steps.
>
> Graham
>
> On 31/10/2014, at 1:12 PM, Kelvin Wong <[email protected] <javascript:>>
> wrote:
>
> I'm sorry to throw too much questions to you. I just want to provide more
> information to make it easier to find the cause.
> Also, I rushed to you because this situation happened during sleep time.
> So I'm little stressed about it.
>
> Back to your question.
> After following your instructions to setup mod_wsgi plugin to monitor the
> mod_wsgi processes in New Relic, I take a look on the data.
> I found that it seemed normal to me. Before the 503 situation, "Average
> Memory Used (bytes/process)" and "Process Count" do not have great changes.
> But looking data from "Server", the Apache processes still accumulating.
> So, can we conclude that Apache child worker processes growing in memory?
>
> Also, as I post previously, the main Apache error log,
> [Wed Oct 29 12:56:26.727197 2014] [mpm_worker:error] [pid 1322:tid
> 139958218430336] AH00287: server is within MinSpareThreads of
> MaxRequestWorkers, consider raising the MaxRequestWorkers setting
> [Wed Oct 29 12:56:30.730902 2014] [mpm_worker:error] [pid 1322:tid
> 139958218430336] AH00286: server reached MaxRequestWorkers setting,
> consider raising the MaxRequestWorkers setting
>
> I changed the config accordingly, the 503 situation still happened.
> <IfModule mpm_worker_module>
> StartServers 2
> MinSpareThreads 25
> MaxSpareThreads 75
> ThreadLimit 64
> ThreadsPerChild 25
> MaxRequestWorkers 400
> MaxConnectionsPerChild 0
> </IfModule>
>
>
> Thanks.
>
>
> On Friday, October 31, 2014 3:30:58 AM UTC+8, Graham Dumpleton wrote:
>>
>> Before I can help you, you need to start answering my questions. If you
>> pepper me with email after email on different things and don't answer the
>> questions I ask you, then we will get no where. When I ask questions it is
>> because that information is likely key to me being able to understand the
>> problem and help you.
>>
>> The main question I put to you was about how you are determining which
>> processes are growing in memory.
>>
>> It needs to be properly determined whether the processes which were
>> growing in memory were Apache child worker processes or mod_wsgi daemon
>> processes.
>>
>> The reason for this is that if they are in fact mod_wsgi daemon
>> processes, then you are possibly in part causing the problem yourself by
>> setting MaxConnectionsPerChild. That setting has got nothing to do with
>> the mod_wsgi daemon processes and by setting it, you are creating the
>> conditions which cause the sorts of errors you are seeing.
>>
>> So, which processes are growing in memory?
>>
>> Graham
>>
>> On 31/10/2014, at 12:40 AM, Kelvin Wong <[email protected]> wrote:
>>
>> I found that on the main Apache log, when it is occurring, Apache is
>> doing graceful restart.
>> How can I avoid Apache graceful restart?
>>
>> Thanks.
>>
>>
>> On Tuesday, October 28, 2014 1:30:06 PM UTC+8, Graham Dumpleton wrote:
>>>
>>> Would suggest upgrading to mod_wsgi 4.3.0 if you can as the error
>>> messages when there are communication problems between Apache child worker
>>> process and mod_wsgi daemon process have been improved.
>>>
>>> More comments below.
>>>
>>> On 28 October 2014 15:43, Kelvin Wong <[email protected]> wrote:
>>>
>>>> Hi Graham and everyone else
>>>>
>>>> I'm running multiple site on Django 1.6.7, Apache/2.4.7 (Ubuntu 14.04),
>>>> OpenSSL/1.0.1f, mod_wsgi/4.2.5, Python/2.7.6, Server MPM: worker.
>>>> I found that the server start returning 504 and then 503, and the
>>>> following error shown up.
>>>> I researched some issues related with it, even added "WSGISocketPrefix
>>>> /var/run/apache2/wsgi", but the issue still occured.
>>>> I have no idea why it happened. Can anyone give some directions on this
>>>> issue?
>>>>
>>>> Thanks!
>>>>
>>>> apache error log
>>>> [Sun Oct 26 07:34:34.732934 2014] [wsgi:error] [pid 29268:tid
>>>> 140053011478272] [client xx.xxx.xxx.xxx:xxxxx] Timeout when reading
>>>> response headers from daemon process 'site-1': /home/ubuntu/site-1/
>>>> apache/wsgi.py
>>>> [Sun Oct 26 07:34:37.198806 2014] [wsgi:error] [pid 27816:tid
>>>> 140052910765824] (11)Resource temporarily unavailable: [client xx.xx.xx
>>>> .xx:xxxxx] mod_wsgi (pid=27816): Unable to connect to WSGI daemon
>>>> process 'site-1' on '/var/run/apache2/wsgi.17227.2.3.sock'.
>>>>
>>>
>>> This one can occur when the mod_wsgi daemon process crashes. There
>>> should be a segmentation fault error message or similar in the main Apache
>>> error log (not VirtualHost specific log).
>>>
>>> It can also occur if there are incomplete requests still running when a
>>> mod_wsgi daemon process is shutdown on being restarted due to the WSGI
>>> script file being touched or if Apache was restarted. In the latter case,
>>> the mod_wsgi daemon process would have had to have been killed off by
>>> Apache before the Apache child worker process which was proxying it to had.
>>> This can especially be the case if an Apache graceful restart was being
>>> done.
>>>
>>>
>>>> occasionally
>>>> [Tue Oct 28 02:20:40.722140 2014] [wsgi:error] [pid 24158:tid
>>>> 140182690981632] (2)No such file or directory: [client 24.171.250.159:
>>>> 60769] mod_wsgi (pid=24158): Unable to connect to WSGI daemon process
>>>> 'snaptee-production-api-ssl' on '/var/run/apache2/wsgi.30188.7.3.sock'.
>>>>
>>>
>>> This can also be due to Apache graceful restart being done and there
>>> were keep alive connections being handled from a HTTP client. In an Apache
>>> graceful restart, because of Apache handles the mod_wsgi daemon processes,
>>> they don't have a graceful shutdown in the same way as Apache child worker
>>> processes.
>>>
>>> So what happens is the following:
>>>
>>> 1. Apache graceful restart is triggered.
>>> 2. Apache parent process sends SIGUSR1 to Apache child worker process to
>>> signal graceful shutdown.
>>> 3. Apache parent process sends SIGINT to mod_wsgi daemon processes to
>>> signal shutdown.
>>> 4. The mod_wsgi daemon processes complete their requests and restart. In
>>> the next incarnation of the mod_wsgi daemon processes after an Apache
>>> restart they expect a different path for the proxy socket, with the number
>>> at the end increasing based on Apache generation number.
>>> 5. The Apache child worker process because it was in a graceful restart
>>> mode, operates on the understanding that it can keep handling any requests
>>> on a keep alive socket connection from a HTTP client until there are no
>>> more. It therefore takes next request on same connection and tries to
>>> connect to mod_wsgi daemon process, but using the proxy socket name as was
>>> used before, but that name has changed for the next Apache configuration
>>> generation and no longer exists, thus it fails.
>>>
>>> The name of the proxy socket changes across Apache restarts because
>>> otherwise you could have Apache child worker processes under an old
>>> configuration sending requests to a mod_wsgi daemon process using the new
>>> configuration, which could cause problems including security issues. There
>>> are therefore specific protections in place to ensure that only Apache
>>> child worker processes and mod_wsgi daemon mode processes created against
>>> the same Apache configuration generation can talk to each other.
>>>
>>>
>>>> wsgi config for that site
>>>> WSGIDaemonProcess site-1 display-name=site-1 user=www-data threads=25
>>>> python-path=/home/ubuntu/site-1/django:/home/ubuntu/.virtualenvs/site-1
>>>> /lib/python2.7/site-packages
>>>> WSGIProcessGroup site-1
>>>> WSGIApplicationGroup %{GLOBAL}
>>>> WSGIScriptAlias / /home/ubuntu/site-1/apache/wsgi.py
>>>>
>>>> worker.conf
>>>> <IfModule mpm_worker_module>
>>>> StartServers 2
>>>> MinSpareThreads 25
>>>> MaxSpareThreads 75
>>>> ThreadLimit 64
>>>> ThreadsPerChild 25
>>>> MaxRequestWorkers 150
>>>> MaxConnectionsPerChild 1000
>>>> </IfModule>
>>>>
>>>
>>> So my best guess is that you are doing Apache graceful restarts when
>>> these are occurring.
>>>
>>> Are you using Apache graceful restarts as suspected?
>>>
>>> Graham
>>>
>>>
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