Accessing Resources for the First Time in Xamarin.Android is Slow - xamarin

Question

I am creating an application that will target the Android OS using Xamarin (currently Xamarin.Forms but could also be Xamarin.Android). This application will access core business functionality via web API calls.
I have noticed that each time I run the application, the first time I perform an API call it is much slower than it is when making subsequent calls. For the hardware I am targeting, the first calls usually takes somewhere around 3000ms while subsequent calls are somewhere between 200ms and 300ms. This behavior is consistent each time I restart the application.
In trying to isolate where the issue originates, I have tested the API endpoint using Fiddler and the response times are always very fast. I also created a UWP project to see if it was specific to Android or anything Xamarin targets. It looks like this issue is specific to Android.
When the first API call is made, output in the debug window similar to the following is generated:
07-28 00:03:50.354 D/Mono (15670): DllImport searching in: '__Internal' ('(null)').
07-28 00:03:50.354 D/Mono (15670): Searching for 'java_interop_jnienv_call_float_method_a'.
07-28 00:03:50.354 D/Mono (15670): Probing 'java_interop_jnienv_call_float_method_a'.
07-28 00:03:50.354 D/Mono (15670): Found as 'java_interop_jnienv_call_float_method_a'.
07-28 00:03:50.564 D/Mono (15670): Loading reference 8 of netstandard.dll asmctx DEFAULT, looking for System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
07-28 00:03:50.564 D/Mono (15670): Image addref System.Net.Http[0x65db9f70] (asmctx DEFAULT) -> System.Net.Http.dll[0x65dc4118]: 2
07-28 00:03:50.564 D/Mono (15670): Prepared to set up assembly 'System.Net.Http' (System.Net.Http.dll)
07-28 00:03:50.564 D/Mono (15670): Assembly System.Net.Http[0x65db9f70] added to domain RootDomain, ref_count=1
07-28 00:03:50.564 D/Mono (15670): AOT: image 'System.Net.Http.dll.so' not found: (null)
07-28 00:03:50.574 D/Mono (15670): AOT: image '/Users/builder/jenkins/workspace/archive-mono/2019-02/android/release/sdks/out/android-armeabi-v7a-release/lib/mono/aot-cache/arm/System.Net.Http.dll.so' not found: (null)
07-28 00:03:50.574 D/Mono (15670): Config attempting to parse: 'System.Net.Http.dll.config'.
07-28 00:03:50.574 D/Mono (15670): Config attempting to parse: '/Users/builder/jenkins/workspace/archive-mono/2019-02/android/release/sdks/out/android-armeabi-v7a-release/etc/mono/assemblies/System.Net.Http/System.Net.Http.config'.
07-28 00:03:50.574 D/Mono (15670): Assembly Ref addref netstandard[0x64a97cd0] -> System.Net.Http[0x65db9f70]: 2
07-28 00:03:50.574 D/Mono (15670): Loading reference 0 of System.Net.Http.dll asmctx DEFAULT, looking for mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
07-28 00:03:50.574 D/Mono (15670): Assembly Ref addref System.Net.Http[0x65db9f70] -> mscorlib[0x62b37138]: 33
Loaded assembly: System.Net.Http.dll [External]
07-28 00:03:50.584 D/Mono (15670): Loading reference 1 of System.Net.Http.dll asmctx DEFAULT, looking for System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
07-28 00:03:50.584 D/Mono (15670): Assembly Ref addref System.Net.Http[0x65db9f70] -> System[0x645da088]: 6
Thread started: #2
07-28 00:03:50.844 D/Mono (15670): Loading reference 2 of System.Net.Http.dll asmctx DEFAULT, looking for System.Core, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
07-28 00:03:50.844 D/Mono (15670): Assembly Ref addref System.Net.Http[0x65db9f70] -> System.Core[0x6488b0c0]: 6
Thread started: <Thread Pool> #3
Thread started: <Thread Pool> #4
Thread started: <Thread Pool> #5
Thread started: <Thread Pool> #6
Thread started: <Thread Pool> #7
Thread started: <Thread Pool> #8
07-28 00:03:51.265 D/Mono (15670): DllImport attempting to load: 'libmono-native.so'.
07-28 00:03:51.275 D/Mono (15670): DllImport loaded library './libmono-native.so'.
07-28 00:03:51.275 D/Mono (15670): DllImport searching in: 'libmono-native.so' ('./libmono-native.so').
07-28 00:03:51.275 D/Mono (15670): Searching for 'SystemNative_GetNonCryptographicallySecureRandomBytes'.
07-28 00:03:51.275 D/Mono (15670): Probing 'SystemNative_GetNonCryptographicallySecureRandomBytes'.
07-28 00:03:51.275 D/Mono (15670): Found as 'SystemNative_GetNonCryptographicallySecureRandomBytes'.
Thread started: <Thread Pool> #9
On subsequent API calls, nothing is generated in the debug window (which more or less explains why the response times are quicker).
Can someone explain what is happening here? Is there a way around this (say, to load resources up from on application start)? Is there a better way?
I don't think the code that calls the API is the issue (as the UWP version does not exhibit the described behavior) but I have included this code below just in case.
var uri = new Uri("...");
try
{
var stopwatch = new Stopwatch();
stopwatch.Start();
using (var client = new HttpClient())
{
using (var response = await client.GetAsync(uri))
{
}
}
stopwatch.Stop();
ResponseTimeLabel.Text = $"{stopwatch.ElapsedMilliseconds.ToString()}ms";
}
catch (Exception exception)
{
ResponseTimeLabel.Text = $"Exception: {exception.Message}";
}

Answer 1

I search some info, and I guess that there are many reason for this.
The app pool is warming up, routes are being built, DI containers may be registering object graphs, caching is occurring, execution plans are being generated and cached, etc etc etc.
there are also other concerns such as JIT overhead and DNS lookups.
You should always assume application warmup time will be required. It is considered best practice to warm up the application on first run to ensure quick responses for consumers of your application. You can't avoid this warmup cost, nor should you try to do so. Rather, structure your deployments so that you are able to warm up the application before users hit it, such as proper load balancing and server rotation.