Azure App Service "Local Written Bytes" - azure

I have an app service running that has 8 instances running in the service plan.
The app is written in asp dotnet core, it's an older version than is currently available.
Occasionally I have an issue where the servers start returning a high number of 5xx errors after a period of sustained load.
It appears that only one instance is having an issue - which is causing the failed request rate to climb.
I've noticed that there is a corresponding increase in the "locally written bytes" on the instance that is having problems - I am not writing any data locally so I am confused as to what this metric is actually measuring. In addition the number of open connections goes high and then stays high - rebooting the problematic instance doesn't seem to achieve anything.
The only thing I suspect is that we are copying data from a user's request straight into Azure Blob Store using the UploadFromStreamAsync from the HttpRequest.Body - with the data coming from a mobile phone app.
Microsoft support suggested we swapped to using local cache as an option to reduce issues with storage, however this has not resolved the issue.
Can anyone tell me what is the "locally written bytes" actually measuring? There is little documentation on this metric that I can find in google.

Related

Azure WebApps leaking handles "out of nothing"

I have 6 WebApps (asp.net, windows) running on azure and they have been running for years. i do tweak from time to time, but no major changes.
About a week ago, all of them seem to leak handles, as shown in the image: this is just the last 30 days, but the constant curve goes back "forever". Now, while i did some minor changes to some of the sites, there are at least 3 sites that i did not touch at all.
But still, major leakage started for all sites a week ago. Any ideas what would be causing this?
I would like to add that one of the sites does only have a sinle aspx page and another site does not have any code at all. It's just there to run a webjob containing the letsencrypt script. That hasn't changed for several months.
So basically, i'm looking for any pointers, but i doubt this can has anything to do with my code, given that 2 of the sites do not have any of my code and still show the same symptom.
Final information from the product team:
The Microsoft Azure Team has investigated the issue you experienced and which resulted in increased number of handles in your application. The excessive number of handles can potentially contribute to application slowness and crashes.
Upon investigation, engineers discovered that the recent upgrade of Azure App Service with improvements for monitoring of the platform resulted into a leak of registry key handles in application worker processes. The registry key handle in question is not properly closed by a module which is owned by platform and is injected into every Web App. This module ensures various basic functionalities and features of Azure App Service like correct processing HTTP headers, remote debugging (if enabled and applicable), correct response returning through load-balancers to clients and others. This module has been recently improved to include additional information passed around within the infrastructure (not leaving the boundary of Azure App Service, so this mentioned information is not visible to customers). This information includes versions of modules which processed every request so internal detection of issues can be easier and faster when caused by component version changes. The issue is caused by not closing a specific registry key handle while reading the version information from the machine’s registry.
As a workaround/mitigation in case customers see any issues (like an application increased latency), it is advised to restart a web app which resets all handles and instantly cleans up all leaks in memory.
Engineers prepared a fix which will be rolled out in the next regularly scheduled upgrade of the platform. There is also a parallel rollout of a temporary fix which should finish by 12/23. Any apps restarted after this temporary fix is rolled out shouldn’t observe the issue anymore as the restarted processes will automatically pick up a new version of the module in question.
We are continuously taking steps to improve the Azure Web App service and our processes to ensure such incidents do not occur in the future, and in this case it includes (but is not limited to):
• Fixing the registry key handle leak in the platform module
• Fix the gap in test coverage and monitoring to ensure that such regression will not happen again in the future and will be automatically detected before they are rolled out to customers
So it appears this is a problem with azure. Here is the relevant part of the current response from azure technical support:
==>
We had discussed with PG team directly and we had observed that, few other customers are also facing this issue and hence our product team is actively working on it to resolve this issue at the earliest possible. And there is a good chance, that the fixes should be available within few days unless something unexpected comes in and prevent us from completing the patch.
<==
Will add more info as it comes available.

App services on azure seems to be very slow

I am trying to track down when our frontend started to work that slow. Recently I created new app services within the same service plan.
so now I have six apps (2 frontend, 4 backend) running under same App Service plan using Basic pricing tier. Also, we use Kudu for deployments.
Could that be the reason? or how to look for the reason?
this is overview of that service plan
appreciating any ideas and suggestions
#user122222 This is a high CPU issue and not a slow request issue as others have pointed out.
An immediate action you can take is to scale up. If you are using a B1 instance in the basic tier, try to scale up to a B3, which will provide you with more CPU cores and RAM. See if that provides you relief. If so, then you likely need to remain at this instance level. At this point it would also be worth while to analyze your number of requests. You should scale up when you are running many sites or resource intensive sites and you should scale out when you are receiving a high number of requests.
My money is on the fact that you likely have an issue with your code that is causing a deadlock or similar. Your CPU usage graph is stuck at 100% usage over many hours. Even an overloaded ASP will see a few dips over the course of a few hours.
To troubleshoot high CPU usage, start by using the diagnose and solve problems blade in your app service plan. This is the same troubleshooting tool that a support engineer would use in a paid technical support case. Use it to troubleshoot high CPU (not slow requests as based on your screenshot, it would appear the CPU is the culprit of the slow requests).
This can tell you what app in the ASP is causing the issue and sometimes even tell you the process in that app that is causing the issue. Beyond this, I'd suggest creating and analyzing a memory dump of the problematic web app. More steps on how to do that here.
Please try to restart the worker instance.
https://learn.microsoft.com/en-us/rest/api/appservice/app-service-plans/reboot-worker#code-try-0

Multiple Redis connection exception (No Connection available to service) during App service swap slots

I have a web app in production (.Net Core), I deployed it in Azure as App service which is in premium tier p2v2 4 instances. I am also using Azure Redis cache (Premium Tier) which my app is using it as cache. I have two app services (primary and secondary) configured Traffic Manager for load balancing.
Whenever I am trying to deploy my app into production using swap slot feature, Both the app service response time goes up to 20 secs and it is down for around 1 minute and my CPU utilization goes close to 90%. And I am seeing multiple exceptions from Redis client (For ex: No connection is available to service this operation: EVAL; It was not possible to connect to the Redis server(s). To create a disconnected multiplexer, disable AbortOnConnectFail. ConnectTimeout; IOCP: (Busy=0,Free=1000,Min=8,Max=1000), WORKER: (Busy=452,Free=32315,Min=8,Max=32767), Local-CPU: n/a) and my HttpQueue length goes above 10
I can infer from the above image is that worker thread has been overloaded, Donno why it is happening
I am using .Net StackExchange Redis client version 2.0.601, recently did an update from version 1.2.4
Note:
I didn't use slot specific app setting.
It keeps happening for every swap slots during deployment
I didn't find any app service restart in the logs.
I want to know any of you guys are facing this issue, if yes please suggest me where is the problem or how to debug and it would also better if you can share any of things you tried.
I tried to find any error logs in AZure Redis cache server but couldn't find any.
I am trying to figure out what is causing this issue, how to debug this kind of issues with azure, and whether anybody encountered the same and have implemented any resolution for the same?
Please let me know if you need any additional details.
Here is something which might be worth trying :
Cache metrics are reported using several reporting intervals, including Past hour, Today, Past week, and Custom. The Metric blade for each metrics chart displays the average, minimum, and maximum values for each metric in the chart, and some metrics display a total for the reporting interval.
Each metric includes two versions. One metric measures performance for the entire cache, and for caches that use clustering, a second version of the metric that includes (Shard 0-9) in the name measures performance for a single shard in a cache. For example if a cache has 4 shards, Cache Hits is the total amount of hits for the entire cache, and Cache Hits (Shard 3) is just the hits for that shard of the cache.
Try looking for the Error metric while monitoring.
https://learn.microsoft.com/en-us/azure/azure-cache-for-redis/cache-how-to-monitor#available-metrics-and-reporting-intervals
Additionally , we need to retry for TimeoutException, RedisConnectionException or SocketException even which ensure it will try to connect in case of any exception, you can read about all the best practises arouns Redis Cache usage in below doc:
https://learn.microsoft.com/en-us/azure/azure-cache-for-redis/cache-best-practices
https://learn.microsoft.com/en-us/azure/azure-cache-for-redis/cache-best-practices#when-is-it-safe-to-retry
Hope it helps.

AspNet Core on Azure App Service peaks to 100% CPU and app gateway load balancer not working

We have few of our internal business services hosted on an isolated ASE in Azure.
These services run on a medium app service plan with 2 instances.
This environment has been in production and use for little more than a month now and has been performing fairly well apart from the occasional sudden CPU spike to 100% in one of the instance which bring down the services.
We don't have auto scaling setup but have 2 instances running all the time.
The services are `aspnetcore` webapi and the runtime is dotnet core 2.0.
Every time I have come across this issue in the last couple of weeks I have not been lucky enough to login to kudu and get a process dump to investigate further. The business are literally behind my back to get the service up and running as quick as possible and the easiest route is to restart one of the faulting service or swap slots with a pre-prod environment.
Access to the ASE are also restricted from our network and makes it all the more difficult for me to switch to a WiFi and then go through jump boxes to login to kudu, I had asked our Ops engineer to get me the dump when this issue is reported but he has not been listening to me either, mostly for the same reasons as me not able to do it myself.
All exceptions I can see in Application Insights are due to the service themselves going down and there are no exceptions there which can cause the issue in the first place(at least I've not found it yet)
This lead me to take few guess and look for metrics, the only thing raising my
suspicions is garbage collection. I don't see any sudden spike in GC graphs as well, each time the service is re started the graph is fairly a straight line(24 hours) but increases day by day and ends up like below.
But the working memory is a sinusoid graph letting me think there are no memory leaks. But is the above graph over 3 days normal?
The drop is when I restart the service. But all services have a similar trajectory even the one that has not gone down.
I am not sure if this is a problem with an individual service or an environment configuration I have overlooked.
The API endpoints are simple CRUD operations and publish events to a service bus topic after each operation. There is a static `HttpClient` instance used to fetch data from another service. Apart from that there are no unmanaged resources and the DB connections are always wrapped in `using` statements.
I understand I would need a process dump to investigate further but my biggest concern is why is the application gateway(load balancer) not sending traffic to the healthy instance. Because of the gateway going unhealthy cloudflare returns a `502` response to clients using the api.
MS support haven't been able to help and have not answered if we have our load balancers working correctly.
The average number of requests is about 50-60 per minute.
CPU runs at less than 10% apart this sudden surge.
Thanks
It could be that the backend is pegged at 100% CPU and is unable to respond to Application Gateway health probes. When such an issue occurs, were you able to verify, using Backend health logs, the health state of your backends? If both backend instances were unhealthy, it would explain the 502s. If one of them was healthy and responding to probes, then new requests sent to Application Gateway would indeed flow to the healthy instance. If you suspect that is not the case then please reply back with subscription id, gateway name and approximate time window of incident for us to take a look.

Syncing clocks on multiple Azure VMs

I have a requirement to write a load test measuring message transmission latencies. In order to simulate a large number of simultaneous uses without running into thread contention problem on one box, I'm spinning up multiple servers in Azure.
When I got my first results back, I was a little shocked to see that the results indicated the message was received before it was sent. I immediately realized that, while I had an implicit assumption that all the VMs would have their clocks synced to within milliseconds, that was clearly not the case.
I've spent several hours googling ways to resolve this, and I'm not getting anywhere. One thought was to have each VM query the time on a central server using NetRemoteTOD() using a technique similar to this NetRemoteTOD, and then establish a per-machine correction factor to be added to the time measured from the local machine's clock. However when I tried to run that method, I got a error 2184, "The service has not been started" I have verified that both the RPC service and the Windows Time service are running on the both the client and target machines, and I have not been successful in finding any information indicating what other service needs to be running (or even if the error really means what it seems to mean). (I also get the same error when running between my development desktop and a server on our corporate network. However, I can run it successfully to a PDC on the corporate network - but I can't find a PDC on Azure, since neither machine is part of a domain.)
So, does any one have either any information on what service needs to be started to get NetRemoteTOD (or the windows NET TIME command, which relies on NetRemoteTOD under the covers) working. Alternatively, does anyone have a suggestion for some other technique to get a consistent time reference across multiple VMs in Azure? (Note, I don't necessarily need their clocks synced, I just need a way to establish a consistent correction factor to reference the times to a common source. Note also, I need sub-second accuracy - probably about 100 msec will do.) Basically, I just need a windows function or shell command that will get me the time to sub-second accuracy on a given remote server.
Thanks in advance.
PS. Azure servers are running Server 2008 R2 SP1

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