Develop Reference

Windows Phone 8.1 background task closes before completion

My background task takes a long time to complete, and the OS is just killing it. I'm trying to sync my contacts online, here's what I'm doing:
Get all contacts from phonebook (takes ~1 second)
Upload them to a server (~2 seconds)
Retrieve all contacts from server (~2-3 seconds)
Delete all contacts from ContactStore(ContactStore.DeleteAsync sometimes takes 1 minute to complete)
Create a ContactStore and import all contacts )(~1-2 minutes for 1000 contacts)
I have ~100 contacts and it's working well, but I wanted to test with ~1000 contacts and it doesn't complete every time. I'm using a MaintenanceTrigger, but I think it's still too much for a background task, but I need a confirmation for this. MaintenanceTrigger tasks should be allowed to do more resource intensive tasks, so why is the OS killing my background task?

Take a look at this link: https://msdn.microsoft.com/en-us/library/windows/apps/hh202942(v=vs.105).aspx
Resource intensive tasks are constrained to a duration of 10 minutes.
The following constraints must be met before the task is started. If the device stop fulfilling these constraints the agent is terminated immediately.
External power required
Non-cellular connection required
Minimum battery power
Device screen lock required
No active phone call
Cannot change network to cellular
Besides this there are also a memory cap of respectively 11mb and 20mb for low/high end devices.
From your description above the most likely scenario IMO is the memory cap being hit. Maybe this post can help you look into the memory usage of your background task: How to get memory available or used in C#
Key changes to memory limits starting in Windows Phone 8.1 include (found here):
All Windows Phone 8 foreground apps are treated the same. We no longer have different memory caps for XNA, native or Silverlight apps.
Windows Phone 8.1 apps (including both Silverlight 8.1 and Windows Runtime) apps do have slightly higher caps than Windows Phone 8 apps.
Memory caps for all app types, including Continuous Background Execution (CBE), scale up with increased memory.
There is no longer a "default" and "higher" cap - there is only the default cap.
The ID_FUNCCAP_EXTEND_MEM manifest entry is ignored for all apps running on Windows Phone 8.1.
The ID_REQ_MEMORY_300 manifest entry is still valid, but you should really make your app run on all devices.
The new equivalent of ID_REQ_MEMORY_300 is below. This entry should be added to the AppX manifest (not to the WMAppManifest).

Finally, my task got canceled with the reason ExecutionTimeExceeded, so this is the problem. It seems that trying to import ~1000 contacts in the ContactStore takes ~12 minutes, which is too long for a background task. I'll have to make the user to open the app and do the import. Thank you for your help.

erm... may be silly but...
"Background tasks that use a maintenance trigger run only when the system is connected to AC power." Taken from MSDN
Could it be plugged into the mains when it works?, and not plugged in when it doesn't work?
EDIT: Are you considering how busy the phone is when you try to 'sync' contacts? Are you forcing the app to allways run in the background via Battery Saver?
You could do something like this to see how busy your phone is... or it could be the battery saver halting your app if download size etc limits are reached...
Taken from here...
var result = await BackgroundExecutionManager.RequestAccessAsync();
if (result == BackgroundAccessStatus.Denied)
{
// Handle this if it is important for your app.
}
"If the result is denied the phone thinks it has too much background task active. In that case you can prompt your users to go the Battery saver application and force allow your app to run in the background even if the phone don´t want to..."

Run frequent background task on Windows Phone 8.1

Windows Phone platform offers BackgroundTasks infrastructure. I can register my task like that:
builder.TaskEntryPoint = TaskName;
var trigger = new TimeTrigger(15, false);
builder.SetTrigger(trigger);
builder.Register();
The problem is, that TimeTrigger min interval is ~15 minutes, which is pretty large for my application. I need to have task which runs every 1-2 minutes in the background. Is it possible on Windows Phone?

It is not possible to have BackgroundTasks that run on a minute interval basis.
As you said already the minimum interval is 15 minute.
This is an OS limitation to prevent developers to build battery draining applications.
You always have some workarounds, like having a PushNotificationTrigger, and manage to send a push notification to your device every minute. (I guess some people manage to do so with the ScheduledToastNotification), but i wouldn't recomend it.

AFAIK with the official API - it's not possible to run TimeTrigger so often. Note that on WIndows Phone, the interval is even larger (MSDN):
Windows has a built-in timer that runs background tasks in 15-minute intervals. Note that on Windows Phone, the interval is 30 minutes.
I doubt it will be possible, due to battery consuption/restrictions. Maybe you can leave your app in the foreground and disable lockscreen (by using DisplayRequest).
Also you may try to run a timer along with obtaining deferal in your BackgroundTask. I've not tried that, for sure there will be many problems (CPU limit, memory and other restrictions), I'm not sure if that is not against certification requirements and of course it doesn't guarantee that your BackgroundTask wouldn't be terminated by OS.

Background Tasks for regular file syncing

For my app I need to be able to let the user specify a given time and let my app run a background task at that specific time.
I understand that with wp8.1 a timetrigger background task can only run every 15 minutes. Is there anything I can do to ensure my app will run the task if the time has gone 10 minutes over the user's specified time?

For what I understand tasks are only executed in intervals multiple of 15 minutes in Windows 8.1 and 30 minutes in Windows Phone 8.1. Have a look here: http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh977059.aspx

I can not add a comment, that's why I share my experiences as an answer.
We had a similar problem in an application - we target the Windows Store 8.1 platform but our experiences can be relevant for you as the app cycles in the two platform are converged (http://msdn.microsoft.com/en-us/library/windows/apps/dn632424.aspx).
In our app we have to monitor streams and collect information. This logic has to be executed per minute what is not supported, as mentioned. That's why we use a simple Task with a CancellationTokenSource, and we start/stop the task in the app cycle-related events (Suspending, Resuming). This "solution" is not acceptable if the execution of the background logic is required when the app is suspended, too, but in our case this was not required.

Memory consumption on Windows Phone 8 is 3 times what is on Windows Phone 7

I'm developing an app that runs on both WP7 and WP8. After I finished it I started running the Performance analysis to improve the app overall performance.
The app is written for WP7, but I have some projects that run over WP8 to add features like in-app-purchase, lockscreen and so on.
When I ran the memory analysis on WP7, the app averaged 50mb of ram throughout execution, an acceptable value, the weird thing was when I ran the same xap on a WP8 device the app uses on average 150mb of ram.
I searched around the web and found nothing related to this issue.
I used a Lumia 800 and lumia 510 to make the analysis on the WP7., and use a Lumia 520, 820 and 925 to make the analysis on WP8 devices.
I'm using Visual Studio 2012.
I also considered that it could be a bug on VS2012, but the app crashes on the lumia 520 (with out of memory, after a while) but never crashes on the lumia 800 or 510.
The app is heavy on images, but all of them are the exact size that is used on the phone (never more than 50 100x100 jpg pictures are displayed in the app at the same time).
Any suggestions and solutions are welcome.
EDIT: The memory differences that I'm talking about is just by running the Home view, my performance analysis test is simply, launch the app swipe some controls (I have a pivot with 2 items) and close the session. There is no navigation involved. I'm using a RadDataboundListBox, but already tried to change to Listbox and LongListMultiSelector without any significant memory consumption change

Can I periodically run a background task in WP8 without my app running?

I want to potentially update the tile of my WP8 app periodically - no more frequently than every hour or two. However, I don't want my app to have to be running for this background task to take place (will query a WAMS and, based on the returned results, update the WP8 app's tile).
Is this possible, or do background tasks (I found this tutorial on how to create one:
http://thesociablegeek.com/windows-8/livetiles/modifying-live-tiles-in-a-background-process/) require that the app that hosts them be running?
IOW, what I basically need is similar to a Windows Service, that his hosted by the OS, not a/my particular app.

Here's a guide how to use so called Background Agents in Windows Phone 7 or 8:
Background agents for Windows Phone
Basically you can schedule simple task to be executed every 30 minutes, even the main application is not running. There are system requirements on these tasks like memory cap constraint 6 MB for WP7 and 11 MB for WP8 tasks, run time of each task 25 seconds, and limited available APIs.

I would recommend that you code your tile update logic as a periodic task and then configure it to run in background. this way, you don't have to maintain your app running in background.
Hope this helps.