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In this post I will share how to improve User Experience by reducing the number of HTTP 401 and HTTP 304 responses.
In general HTTP 304 is returned by web server when the browser is not really sure about up-to-date’ness of the resource. Imagine this conversation:
This one extra roundtrip might look very subtle in case when there are very few static elements on the page. In case there are many static elements on the page the User Experience can be severely affected. Below is an extreme example of HTTP 304 responses captured by Fiddler. All the images in the diagram are stored in local cache but they never displayed right away – Browser first consults with the server and gets HTTP 304 before displaying it:
HTTP 401 returned when Browser requests a resource that requires authentication. The fact that the resource requires authentication results in two HTTP requests – initial requests gets HTTP 401 asking for credentials, and subsequent request is satisfied with the actual response after the creds were validated. Something similar to this:
Consider the following solution structure:
<?xml version="1.0"?> <configuration> <system.web> <authorization> <allow="*"/> </authorization> </system.web> <location path="Restricted"> <system.web> <authorization> <deny="?"/> </authorization> </system.web> </location> </configuration>
This configuration achieves the following:
A customer complained that his ASP.NET web application gets slow periodically. It happens at random times, the system just gets slow then after few minutes it gets back on track with normal response times.
One of the reasons for such behavior is an AppPool default recycling policy set in IIS.
The default recycling policy for application application pool is 29 hours. Both IIS 6.0 and IIS 7.
It means that every 1740 minutes the application pool recycled. 1740 minutes means 29 hours. 29 hours means randomness.
To make sure you are dealing with recycling (there are few more reasons I will be discussing in the next posts) one needs to correlate IIS recycle events in Windows Event log with slowness of the pages in IIS log.
Start with Windows Event log. Go to System Event Log and filter events with W3SVC as a source. Look for recycle events with id of 1074:
Notice the time the recycle occurred.
Go to IIS logs and filter out the resources that took significantly long. One way to look into the IIS logs is using Excel as outlined here - Identify ASP.NET, Web Services, And WCF Performance Issues By Examining IIS Logs. Another way is using LogParser as outlined here (via Tess) - Using LogParser 2.2 to Parse IIS Logs and Other Logs.
Notice time-taken is pretty lengthy, around 30 seconds. Notice the time it is taken. Now look at the times that the Event log captured 1074 events – it is 2 hours diff. It is because IIS logs events using GMT and Windows Event log is time zone sensitive. I live in GMT+2 time zone. So the times are correlated.
Remove IIS recycling default policy. I once heard someone saying it is training wheels. If your app pretends to be stable then you do not need recycling at all – of any kind. Just remove it all. If you are using recycle there are potentially two reasons, actually one – you have problematic application that misbehaves. It’s either under your control to fix it by changing the code or out of your your control when you buy an app that you cannot fix. Ask for a fix from the vendor. It might be also a good time to review your capacity plan. In any case – recycling is a workaround and not the solution.
Following is a list of performance counters I am usually taking to spot low hanging fruits when measuring ASP.NET performance:
\.Processor\%Processor Time \.NET CLR Memory(*)\Allocated Bytes/sec \.NET CLR Memory(*)\% Time in GC \.NET CLR Exceptions(*)\# of Exceps Thrown / sec \.NET CLR Loading(*)\Current Assemblies
\.NET CLR LocksAndThreads(*)\Contention Rate / sec \.NET CLR LocksAndThreads(*)\Current Queue Length \ASP.NET\Requests Queued \ASP.NET\Request Wait Time \ASP.NET\Requests Current \ASP.NET Applications\Requests In Application Queue \ASP.NET Applications\Pipeline Instance Count \ASP.NET Applications\Requests Executing \ASP.NET Applications\Requests/Sec \Web Service\Current ISAPI Extension Requests
\ASP.NET\Request Execution Time
Detailed explanation about each counter and its significance can be found here: Chapter 15 — Measuring .NET Application Performance
I particularly love the following diagram from the guide. The diagram helps brainstorming while identifying root cause of the performance issues identified using the counters: