1. Sherlock Homepage
A detective story about running
large web services.
Maarten Balliauw
@maartenballiauw

2. VISUG Partners

3. Site unavailable!

4. Site unavailable!
Primary website location unavailable!
No problem: traffic manager in front – pfew!
Secondary location unavailable!
Website down…
Initial investigation & monitoring showed:
Primary & secondary website location instances all up
Machines available individually
Not through traffic manager and Azure load balancer

5. The cause…
Custom Azure load balancer probe
Implementation (StatusService.cs)
<LoadBalancerProbes>
<LoadBalancerProbe name="HTTP" path="/api/status" protocol="http" port="80" />
</LoadBalancerProbes>
return new HttpStatusCodeWithBodyResult(AvailabilityStatusCode(galleryServiceAvailable),
String.Format(CultureInfo.InvariantCulture,
StatusMessageFormat,
AvailabilityMessage(galleryServiceAvailable),
AvailabilityMessage(sqlAzureAvailable),
AvailabilityMessage(storageAvailable),
AvailabilityMessage(searchServiceAvailable),
AvailabilityMessage(metricsServiceAvailable),
HostMachine.Name));

6. How did we find the issue?
Quote from “Mind Hunter” (written by an FBI profiler):
You have to be able to re-create the crime scene in your head. You need
to know as much as you can about the victim so that you can imagine
how she might have reacted. You have to be able to put yourself in her
place as the attacker threatens her with a gun or a knife, a rock, his fists,
or whatever.
You have to be able to feel her fear as he approaches her. You have to be
able to feel her pain. You have to try to imagine what she was going
through when he tortured her. You have to understand what it’s like to
scream in terror and agony, realizing that it won’t help, that it won’t get
him to stop. You have to know what it was like.
http://highscalability.com/blog/2015/7/30/how-debugging-is-like-hunting-serial-killers.html

7. How did we find the issue?
Debugging requires a particular sympathy for the machine. You must be able to run
the machine and networks of machines in your mind while simulating what-ifs based
on mere wisps of insight.
In reality: it was a hunch
Knowing the system you are working on – even by similarity
Empathy for what is going on in that system
Prior experience / insights

8. Sherlock Homepage
A detective story about running
large web services.
Maarten Balliauw
@maartenballiauw

9. Who am I?
Maarten Balliauw
Antwerp, Belgium
Software Engineer, Microsoft
Founder, MyGet
AZUG
Focus on web
ASP.NET MVC, Azure, SignalR, ...
Former MVP Azure & ASPInsider
Big passion: Azure
http://blog.maartenballiauw.be
@maartenballiauw
Shameless self promotion: Pro NuGet - http://amzn.to/pronuget2

10. History and Context

11. A bit of history
Site serves dependencies for .NET developers worldwide
On average good for ~8.000.000 to ~10.000.000 request per day
(333.333/hr or 5.555/min or 92/sec)
Built 4 years ago on top of a SQL database and OData services
Monolithic – site + OData service are the same app
Minor improvements over the years
Some rough times Q2 2015

12. Architecture overview (Q2 2015)
Front end servers (2 regions)
MVC + WCF OData
Search Service
Lucene based Web API
Search Service
(secondary region)
Lucene based Web API
Azure Storage
Lucene index
Download counts
Azure SQL Database
Metadata
Download counts
Jobs VMs
Create index from database
Create stats
Create download count reports

13. Did we solve the
crime?

14. One of the services caused this…
SQL database?
Storage?
Search?
Metrics service?
return new HttpStatusCodeWithBodyResult(AvailabilityStatusCode(galleryServiceAvailable),
String.Format(CultureInfo.InvariantCulture,
StatusMessageFormat,
AvailabilityMessage(galleryServiceAvailable),
AvailabilityMessage(sqlAzureAvailable),
AvailabilityMessage(storageAvailable),
AvailabilityMessage(searchServiceAvailable),
AvailabilityMessage(metricsServiceAvailable),
HostMachine.Name));

15. Log spelunking
Check SQL database logs – we found we had none (fixed now)
Storage – storage statistics seemed stable
Search – no real pointers to issues there
Metrics service – very lightweight and has been stable for months
Start looking around at the crime scene!
IIS logs, event viewer on web servers
Profiling on web servers

16. Profiling the website
demo

17. It could have been
search...
No real evidence though.

18. Profiling the search
service
demo

19. Turns out it was search!
Profiling the search service revealed some things!
SearcherManager.cs checks Lucene index freshness on Get() – MaybeReopen()
StartReopen() blocks access to the index until finished
Part of HTTP request pipeline – blocking request handling
This was fixed by getting these calls out of the HTTP request path.
The suspect no longer had code available – used our informants
www.jetbrains.com/dotpeek

20. Search had some other flaws…
Actually also visible in the dotTrace snapshot we just saw: high GC!
Memory profiling the search service revealed some things! (I lost the actual traces )

21. Search had some other flaws…
High memory traffic on reading download count # for search ranking
The source: DownloadLookup.cs#L18
Fixed by:
Reusing objects (instead of new)
JsonStreamReader instead of JObject.Parse(theWorld)

22. In the meanwhile…
Added additional monitoring
Added additional tracing
Started looking into using AppInsights for better insights into application behavior
Events happening on the website
Requests
Exceptions
Execution and dependency times (basic but continuous profiling)

23. Internal Server (T)Error
during package restore

24. What we were seeing…
On the V2-based feeds:
500 Internal Server Error
during package restore
Response time goes up
while # of requests goes down
EventVwr on servers: lots of IIS crashes
Lots of crash dumps on web servers

25. So IIS crashes… Could it be?
HTTP.SYS tells us when things go wrong
D:WindowsSystem32LogFilesHTTPERR
2015-07-31 01:46:34 - 60810 - 80 HTTP/1.1 GET /api/v2/FindPackagesById()?id='...' - 1273337584
Connection_Abandoned_By_ReqQueue 86cd3cb1-729c-425c-898f-b15b0330bc38
Connection_Abandoned_By_ReqQueue
“A worker process from the application pool has quit unexpectedly or orphaned a pending
request by closing its handle. Specific to Windows Vista and Windows Server 2008.”
Gift from the gods: crash dumps!
C:ResourcesDirectory31edcaa5186f…...DiagnosticStoreWAD0104CrashDumps

26. Analyzing a crash
dump
demo

27. An Exception crashes IIS?
Time to crank up the search engine queries!
Found a similar issue: unobserved task exceptions causing IIS to crash
// If metrics service is specified we post the data to it asynchronously.
if (_config != null && _config.MetricsServiceUri != null)
{
// Disable warning about not awaiting async calls
// because we are _intentionally_ not awaiting this.
#pragma warning disable 4014
Task.Run(() => PostDownloadStatistics(id, version, …));
#pragma warning restore 4014
}

28. TaskScheduler.UnobservedTaskException += (object sender, UnobservedTaskExceptionEventArgs excArgs) =>
{
// ... log it ...
excArgs.SetObserved();
};
Tasks and fire-and-forget are evil!
Unobserved task can cause the entire process to give up on Exception
Handle unobserved task Exceptions!

30. High response times
on the web server

31. What we were seeing…
High response times on the servers
Resulting in higher than normal CPU usage on the servers
Azure would often auto-scale additional instances
Profiling the web application still showed wait times with no obvious cause…

32. Eating donuts

33. Research
Reading and searching on what could be the cause of these issues
http://stackoverflow.com/questions/12304691/why-are-iis-threads-so-precious-as-compared-to-regular-clr-threads
http://www.monitis.com/blog/2012/06/11/improving-asp-net-performance-part3-threading/
https://msdn.microsoft.com/en-us/library/ms998549.aspx
http://blogs.msdn.com/b/tmarq/archive/2007/07/21/asp-net-thread-usage-on-iis-7-0-and-6-0.aspx
https://support.microsoft.com/en-us/kb/821268
Consider minIoThreads and minWorkerThreads for Burst Load
If your application experiences burst loads where there are prolonged periods of inactivity between the burst
loads, the thread pool may not have enough time to reach the optimal level of threads. A burst load occurs
when a large number of users connect to your application suddenly and at the same time. The
minIoThreads and minWorkerThreads settings enable you to configure a minimum number of worker
threads and I/O threads for load conditions.

34. The result
GUESS WHERE WE DID THE TWEAK… COMPARED TO LAST WEEK…

35. Making it permanent
Part of the cloud service startup script
# Increase the number of available IIS threads for high performance applications
# Uses the recommended values from http://msdn.microsoft.com/en-us/library/ms998549.aspx#scalenetchapt06_topic8
# Assumes running on two cores (medium instance on Azure)
&$appcmd set config /commit:MACHINE -section:processModel -maxWorkerThreads:100
&$appcmd set config /commit:MACHINE -section:processModel -minWorkerThreads:50
&$appcmd set config /commit:MACHINE -section:processModel -minIoThreads:50
&$appcmd set config /commit:MACHINE -section:processModel -maxIoThreads:100
# Adjust the maximum number of connections per core for all IP addresses
&$appcmd set config /commit:MACHINE -section:connectionManagement /+["address='*',maxconnection='240'"]

36. Package restore
timeouts

37. What we were seeing…
On the V2-based feeds:
Package restore timeouts coming from the WCF OData service
Occurs every 7-15 hours, fixes itself ~15 minutes later
Extreme load times on Get(Id=,Version=) – probably the cause of these timeouts

38. No easy way to reproduce…
Happening only on production
Observation after RDP-ing in: 100% CPU when it happens
No way to profile continuously – AppInsights did show us the entry point
Donut time again
The thing we recently changed was minIOThreads and throughput
The slow code path is FindPackagesById()
Makes HTTP calls to search service
What could this setting and HTTP calls have in common…

39. HttpClient, Async and multithreading
Interesting article benchmarking HttpClient in async and multithreading scenarios
Async + HttpClient are not limited in terms of concurrency
Many CPU’s and threads? Many HttpClients and requests
Many HttpClients and requests? Many TCP ports used on machine
Many TCP ports used? TCP port depletion
But aren’t ports reclaimed?
240 seconds TIME_WAIT (4 minutes)
Users also use up TCP ports
“As far as HTTP requests are concerned, a limit should always be set to
ServicePointManager.DefaultConnectionLimit. The limit should be large enough to allow a
good level of parallelism, but low enough to prevent performance and reliability problems (from the
exhaustion of ephemeral ports). “

40. Limiting HttpClient async concurrency
Set ServicePointManager properties on startup
Nagling – “bundle traffic in properly stuffed TCP packets”
Expect100Continue – “only send out traffic if server says 100 Continue”
Both optimizations also disabled
// Tune ServicePointManager
// (based on http://social.technet.microsoft.com/Forums/en-US/windowsazuredata/thread/d84ba34b-b0e0-
4961-a167-bbe7618beb83 and https://msdn.microsoft.com/en-
us/library/system.net.servicepointmanager.aspx)
ServicePointManager.DefaultConnectionLimit = 500;
ServicePointManager.UseNagleAlgorithm = false;
ServicePointManager.Expect100Continue = false;

41. Some charts…

42. Memory pressure

43. What we were seeing…
Massive memory usage! Even when changing VM sizes.
100% of memory on a Medium Azure instance
100% of memory on a Large Azure instance
100% of memory on a X-Large Azure instance

44. What is eating this memory?
Memory profiling!
On the server?
Try to reproduce it?
Decided on the latter

45. Reproducing
production traffic
demo

46. .NET Memory Management 101
Memory Allocation
.NET runtime reserves region of address space for every new process
managed heap
Objects are allocated in the heap
Allocating memory is fast, it’s just adding a pointer
Some unmanaged memory is also consumed (not GC-ed)
.NET CLR, Dynamic libraries, Graphics buffer, …
Memory Release or “Garbage Collection” (GC)
Generations
Large Object Heap

47. .NET Memory Management 101
Memory Allocation
Memory Release or “Garbage Collection” (GC)
GC releases objects no longer in use by examining application roots
GC builds a graph that contains all the objects that are reachable from these
roots.
Object unreachable? GC removes the object from the heap, releasing memory
After the object is removed, GC compacts reachable objects in memory.
Generations
Large Object Heap

48. .NET Memory Management 101
Memory Allocation
Memory Release or “Garbage Collection” (GC)
Generations
Managed heap divided in segments: generation 0, 1 and 2
New objects go into Gen 0
Gen 0 full? Perform GC and promote all reachable objects to Gen 1. This is typically pretty fast.
Gen 1 full? Perform GC on Gen 1 and Gen 0. Promote all reachable objects to Gen 2.
Gen 2 full? Perform full GC (2, 1, 0). If not enough memory for new allocations, throws
OutOfMemoryException
Full GC has performance impact since all objects in managed heap are verified.
Large Object Heap

49. .NET Memory Management 101
Memory Allocation
Memory Release or “Garbage Collection” (GC)
Generations
Large Object Heap
Generation 0 Generation 1 Generation 2
Short-lived objects (e.g. Local
variables)
In-between objects Long-lived objects (e.g. App’s
main form)

50. .NET Memory Management 101
Memory Allocation
Memory Release or “Garbage Collection” (GC)
Generations
Large Object Heap
Large objects (>85KB) stored in separate segment of managed heap: Large
Object Heap (LOH)
Objects in LOH collected only during full garbage collection
Survived objects in LOH are not compacted (by default). This means that LOH
becomes fragmented over time.
Fragmentation can cause OutOfMemoryException

51. The .NET garbage collector
Simulates “infinite memory” by removing objects no longer needed
When does it run? Vague… But usually:
Out of memory condition – when the system fails to allocate or re-allocate memory
After some significant allocation – if X memory is allocated since previous GC
Failure of allocating some native resources – internal to .NET
Profiler – when triggered from profiler API
Forced – when calling methods on System.GC
Application moves to background
GC is not guaranteed to run
http://blogs.msdn.com/b/oldnewthing/archive/2010/08/09/10047586.aspx
http://blogs.msdn.com/b/abhinaba/archive/2008/04/29/when-does-the-net-compact-framework-garbage-collector-run.aspx

52. Analyzing memory
usage
demo

53. So our DI container? NInject?
Our memory profiling confirms it. The retained EntitiesContext also
retains entities and SQL connections.
Spelunking the NInject source code, we found the
GarbageCollectionCachePruner responsible for releasing objects.
Runs every 30 seconds (timer)
Releases objects only if GC happened in that time
GC is not guaranteed to run, so NInject potentially never releases objects
Known, old bug.
https://groups.google.com/forum/#!topic/ninject/PQNMIsQhCvE
http://stackoverflow.com/questions/16775362/ninject-caching-object-that-should-be-disposed-memoryleak

54. Replacing our DI container (Autofac)
Perform replacement
Run same analysis on new codebase
and verify objects are freed
Once deployed:
Immediate drop in response times
Memory usage now stable at ~4 GB

55. Conclusion

56. Conclusion
Debugging requires a particular sympathy for the machine. You must be
able to run the machine and networks of machines in your mind while
simulating what-ifs based on mere wisps of insight.
Bugs hide. They blend in. They can pass for "normal" which makes them tough to find.
One bug off the streets doesn’t mean all of them are gone. Sometimes one gone exposes
another.
Know your system, know your tools, know your options. Look for evidence.
Profilers (performance and memory), dump files, AppInsights and others
Dive in. It builds experience and makes solving the next crime scene easier.
https://msdn.microsoft.com/en-us/library/ee817663.aspx

57. Thank you!
http://blog.maartenballiauw.be
@maartenballiauw
http://amzn.to/pronuget2

58. VISUG Partners