Kevin Gosse
@KooKiz
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Bluesky: @kevingosse .net Software developer at Datadog, passionate about .NET, performance, and debugging. Microsoft MVP.
Paris / France
Joined January 2009
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My typical workflow as a performance enthusiast:
1. Write the naive solution to a problem
2. Optimize it
3. Benchmark both solutions
4. Pretend it never happened
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We're in the middle of migrating one of our biggest apps (5000+ servers) to .net core. In yellow is CPU usage for .net framework + . In green is .net core 3.1 + aspnetcore. We've migrated other apps but it's the first time we see such a dramatic improvement
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In preparation for my talk at
@update_conf
, I've prepared a tool to visualize the activity of the GC. It was initially meant as a gimmick, but it turns out to be a good way to understand some GC mechanisms.
For instance, here you can see the effect of
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While reading Stephen Toub's article on .NET 8 performance, I noticed a very interesting feature introduced in .NET 8, that I never heard about: [UnsafeAccessor].
It allows you to get a reference to a private field or method of a target type while ignoring visibility checks.
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I recently compiled a list of best practices in C# for Criteo, so I thought it would be interesting to share it publicly:
This list is backed by experience: *all* of those items have been spotted at some point in our code and have caused production issues
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I'm happy to finally announce the availability of this tool, under the name of GC (GummyCat) :
It is mainly meant as an educative tool to understand how the memory is organized by the .NET GC (Garbage Collector). It has helped me a lot during the writing
In preparation for my talk at
@update_conf
, I've prepared a tool to visualize the activity of the GC. It was initially meant as a gimmick, but it turns out to be a good way to understand some GC mechanisms.
For instance, here you can see the effect of
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Being able to write C# scripts from WinDbg is really a game changer when manipulating a memory dump with complex data structures
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You know this box of old cables that we all have? Two years ago I threw away the ones I would *really* never need. Among them, IDE cables. Completely useless nowadays.
And of course, today, I need an IDE cable.
Learn from my mistake, don't throw away anything from The Box.
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@Marie_mja77
@Linguisticae
La même. Avis de passage dans la boîte aux lettres disant que le colis n'a pas pu être livré. J'appelle le numéro, on me dit que le livreur n'a pas trouvé l'adresse.
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The year is 2022, and I'm debugging a Linux coredump in Windbg because I failed to open it properly with LLDB and GDB
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Regardless of the circumstances around this code, I find the reactions very interesting. I immediately thought "you can do better with a loop". But then I realized that you can understand this code with a glance, while the loop would take you a significant time to understand.
The Dutch government was forced to release the source code of their DigiD digital authentication iOS app. It is written in C#.
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Yesterday I spent the day investigating why calling a C++ library from another was crashing my app. Turns out one was compiled in debug and the other in release, and in that case std::string has a different binary layout. The more I learn about other languages, the more I love C#
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When you add some asynchronous code to a method and have to choose between updating the 120 callers or adding a tiny .GetAwaiter().GetResult()
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I tried solving old
#AdventOfCode
puzzles, but apparently Github Copilot has been trained on so many solutions that it solves it before I can even type anything. Not even the instructions.
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It freaks me out how unreliable the search has become in Visual Studio. That's one of the few areas it is *not* acceptable to mess up.
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We tried fixing in our internal CLR fork, and saw a dramatic improvement in our p99 response time. We will push the fix upstream in the coming days, expect some benefits if you run a memory-constrained .net core app on Linux
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First time actually using Time Travel Debugging, and loving it. I was unable to understand how a particular value was ending up in memory, with TTD it's a matter of setting what memory range you're interested in and you can then browse each memory access with their callstack 🤯
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If for some reason you feel like the performance of your .NET application is affected by lunar cycles, you might want to read this: 😃
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Just blogged: Have some fun with .net core startup hooks
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Time to make it official. I'm leaving Criteo and starting today at
@datadoghq
. I'm joining the .NET APM team. This will probably give me plenty of new performance-related material to talk about 🙂
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Just published an article digging deep into how arrays of doubles are aligned in
#dotnet
. With juicy bits about the allocation context, fake objects, and C# code that you definitely shouldn't deploy in production.
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Just published part 3 of my ongoing series about how to write a .NET profiler in C#:
This time, we write a source generator to automatically generate the code of the wrapper that we use to expose the objects to the native side.
#dotnet
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Just blogged: using ClrMD and NativeAOT to write native WinDbg extensions in C#.
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@Raye_Alex
@GendyGentil
Seule l'assurance responsabilité civile est obligatoire. C'est celle qui couvre les dégâts que tu infligerais à d'autres usagers. Car malheureusement en voiture ce n'est pas toujours celui qui fait de la merde qui subit.
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Working at Datadog, it's also writing C-like C# code that crashes on purpose.
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A reminder as I just got bitten by this: using a new Random instance per thread is NOT safe on .net framework. The seed is generated from the system clock, with a very low resolution, so there is a high risk that many threads are going to generate the same sequence of values
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I just published a new article: "Exploring .NET frozen segments", where we look at how to allocate managed objects outside of the managed heap.
You most definitely can, but ask yourself if you should.
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Just blogged: debugging and fixing the Twitch desktop client. An article where I exhaustively retrace the steps behind an investigation, hoping to describe my mindset when debugging an application I know nothing about.
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Just blogged: how to use a startup hook to easily collect GC metrics from any .net core application
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In case you missed it, Visual Studio has a new feature to show the layout of a C++ object. It's *really* convenient when doing some interop between native and managed code.
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"aaaaaaaaaz".StartsWith("i")
"aaaaaaaaa-".StartsWith("i")
Can you spot the difference? Well, the latter is 2 orders of magnitude slower on Linux.
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I published the first part of a series of articles about how to write a .NET profiler in C#, thanks to NativeAOT:
I know I was supposed to write an article about the LLDB stuff, but I couldn't resist the temptation to first write a proof of concept of a .NET profiler written in C# thanks to NativeAOT
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That's the 5th bug found in the CLR while writing Pro .NET Memory Management 2nd Edition (
#progc2
) 🤓
Thanks
@KooKiz
for having investigated why I was triggering 2 garbage collections while building the upcoming dotnet-fullgc CLI tool -
#progc2
@konradkokosa
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When I wrote the cats stock market app for the Effective Debugging workshop, I really didn't expect to see it featured in so many Visual Studio articles 😺
In case you're curious, the source code is available here:
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Just blogged: Reading .NET performance counters without the PerfCounter API
Or how to skip the middle-man when trying to read perf counters from within the same process.
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After 5 years of nominations from
@lancewmccarthy
and
@theothernt
, it happened when I expected it the least. Thanks to everybody who made this possible.
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I just published a new article, about how to walk the managed heap from your
#dotnet
application:
It only works with server GC, and is extremely unsafe. But it's also a ton of fun.
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@SwiftOnSecurity
In the same vibe, it's been a few months that whenever I click on "Update and shutdown", my desktop computer reboots at the end of the updates and stays on...
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In case you missed it, .NET is getting a Task.WaitAsync API:
Before:
await Task.WhenAny(task, Task.Delay(timeout, token))
After:
await task.WaitAsync(timeout, token)
Nice addition, though I'm still not fan of exception-based cancellation
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TIL: If a task has 2 await continuations, both are inlined and the second one will wait for the first one to complete. That behavior has been fixed here:
This program will never complete on .net framework and .net core < 3.0:
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TIL: The .net core implementation of Socket on Linux does not use I/O threads, and instead relies on ThreadPool.UnsafeQueueUserWorkItem. That's *very* bad for us, because it competes with the application to get threads to complete I/Os, and worsens our thread starvation problems
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Good old Docker on Windows. Note that I didn't even start a container.
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I really, really love C# 12 collection expressions. To the point that I've started wishing that there was a ..= operator 😅
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@ShinyQuagsire
Nice! I also made a program to change the input source of my LG screen (27GL850B), but I took a different approach and decompiled the OnScreen Control application to understand the protocol, then used named pipes to send my commands to OSCApplicationManager
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I published part 4 of my ongoing series of articles about how to write a .NET profiler in C#:
This time we learn how to invoke methods from a native virtual object, and write a source generator to automate the boilerplate code.
#dotnet
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Mono.Cecil is powerful but some parts are tricky to get right, especially when dealing with generics.
I just discovered , that shows you how to generate the given C# code with Cecil. Incredibly useful, I can't believe I never heard of it before.
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Trivia: just decoded the four messages on
@joebelfiore
's shirt during
#build2015
:
http://t.co/zJR35WZFUd
#wpdev
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There are few things I enjoy as much as digging into a new codebase and methodically implementing all those optimizations. Expect a nice speedup for the Datadog .net tracer in the coming versions!
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I had a discussion a while ago with coworkers about how Windows is missing a sudo equivalent, and it turns out there are pretty good alternatives available. I've been using gsudo for a few days now and it works great. I definitely recommend it.
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Preparing a Perf top written entirely in .NET Core, for my part in
@chnasarre
's talk at DotNext
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I found this code in the .NET BCL while studying different strategies to dynamically invoke an object constructor, and wow... My head hurts 😅
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I just learned two things: p/invokes can be declared as local functions, and local functions are scoped to the current block (somehow I assumed they were scoped to the enclosing method).
Included: a spoiler of my next article.
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Just blogged: Why function pointers can’t be used on instance methods? Or an exploration of the CLR ABI to understand why using function pointers on instance methods would require special support.
#dotnet
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Ready to talk about memory management in
#dotnet
at
@update_conf
. Eager to see in what creative ways
@chnasarre
will make the demos fail.
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Just wrote a short guide explaining how to write debugger extensions that use ClrMD and are compatible with WinDbg on Windows *and* LLDB on Linux:
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I got inspired by the
@dotnetosorg
webinar on C# 9.
Introducing to you... An early proof-of-concept of using source-generators and ClrMD to generate strongly-typed entities from a memory dump at compilation time, for easier inspection
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That sounds fun, maybe we could do the same for
#dotnet
📢 "The One Billion Row Challenge"
How fast can YOU aggregate 1B rows using modern
#Java
? Grab your threads, flex your SIMD, and kick off 2024 true coder style by joining this friendly little competition. Submissions accepted until Jan 31.
👉
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@sebify
Professionally we tend to reward complexity: it's much easier to demonstrate your skills by writing complex code that only you can understand, rather than a simple solution to the same problem. The latter is much harder to come up with, but feels obvious to the reader.
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Small teaser for my next article, dumping the managed heap in C# without using ClrMD or the debugging API
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Just blogged: dumping stack objects with ClrMD
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Just in time for the new year, I've migrated my blog from medium to github pages:
Feedback is welcome, especially if you notice something wrong or broken links.
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I don't know if there's a better feeling in my job than that moment when you finally identify a bug in the GC after weeks of investigation 😊
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When developing for aspnetcore/kestrel, keep in mind that the same HttpContext instance is reused for all the requests on a given HTTP connection (in case of keep-alive). If not, in the worst case scenario, you might end-up replying to the wrong request:
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Just blogged on
@CriteoEng
: Optimizing the WinDbg DML parser. Or how using the wrong class at the wrong place can completely ruin your performance
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Nothing new, but I believe this is a little known quirk of C#. Can you predict the output of this code?
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I know I was supposed to write an article about the LLDB stuff, but I couldn't resist the temptation to first write a proof of concept of a .NET profiler written in C# thanks to NativeAOT
3 years later, I'm experimenting with another approach that doesn't require the use of the "LoadManaged" meta plugin to work with LLDB, thanks to NativeAOT
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If you like debugging, you never get a chance to be bored at Criteo. Today, a server unresponsive with 100% CPU usage, 200 threads stuck on a static lock in Exception.RestoreExceptionDispatchInfo. I especially like the comment on the static lock in the BCL 🙃
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I'll be giving a talk at NDC Oslo next month: Designing and rewriting asynchronous tasks from scratch
The concept is to start from nothing an re-implement a future/promise pattern in C#, to understand the complexity and the technical choices behind the TPL
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While working on
#progc2
, to test my understanding of regions, I imagined how the .NET GC would accommodate the allocation of an array bigger than 4MB, when setting a custom LOH threshold to 8MB.
Why would that be interesting? When regions are enabled (by default in .NET 7 and
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Another Linux core dump that I didn't manage to open with gdb, but windbg handled it flawlessly.
@timmisiak
have you considered porting windbg to Linux and calling it wingdb?
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Just blogged on
@CriteoEng
an article about threadpool starvation in
#dotnet
, and how the threadpool queuing algorithm makes the situation even worse:
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I published a write-up of the investigation:
A story about advanced debugging when printf is the only sane tool at your disposal.
Segmentation fault in libmscordaccore. I guess it's time to debug the debugger 😬
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When we realized we could build custom versions of the CLR tweaked for our needs, things got out of hand pretty quickly 😅
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Today in our show, "Seasoned C# developers have to write some C and do everything by hand because of how poor libc is"
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@chrismckee
@update_conf
I admit, "something that looks like a defrag tool" that was the original goal 😅
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I couldn't stop here, I had to try using NativeAOT to write a .NET GC in C# (here, just a conversion of
@konradkokosa
bump pointer GC)
I know I was supposed to write an article about the LLDB stuff, but I couldn't resist the temptation to first write a proof of concept of a .NET profiler written in C# thanks to NativeAOT
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- Spend days researching.
- Finally got the full picture, ready to write an article.
- While writing, test an edge case to validate my understanding.
- Get unexpected results. Discover a whole new mechanism. Article is postponed.
- Spend days researching.
- Rinse, repeat.
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For a while I was satisfied with Medium, but I feel like they crossed a line.
Konrad is migrating his blog to Github Pages, so maybe I'll do the same.
As a reader, what's your take on Medium? I've seen on twitter and reddit that some people are really bothered by it.
Migrating blog from WordPress. Wish me luck. ETA ~1 month.
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The automatic decompilation feature in VS is nice, but when ctrl+clicking on a built-in .NET type it only shows the metadata. Anybody knows how to show the actual implementation, like with Resharper?
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On the importance of continuous performance measurement. We detected a performance regression on our product and were able to easily track down the exact commit that caused it. Detection and diagnosis took about 30 minutes.
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Just blogged: Or how to divert the async machinery to remove boilerplate code from your desktop applications
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It was hard to keep the secret. I'm really hyped to see Ozcode joining Datadog. I'm looking forward to working with those amazing people who share my passion for debugging😃
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Just published the third and last part of my investigation about diagnosing an InvalidProgramException from a memory dump:
Digging into the structure of an IL method, and finding a few bugs in SOS while doing so
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Just blogged: An unconventional way of investigating a NullReferenceException, or how a crash on ARM64 led me to dig into interface dispatch in .NET.
#dotnet
#debugging
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By the way, I'll be at
@DotNextConf
next week in Moscow to talk about -you guessed it- debugging.
For those who can't attend, DotNext talks are usually recorded and available a few weeks after the conference.
For the others, see you there!
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