[jsc-dev] Thread naming policy in WebKit

[Yusuke SUZUKI]

Hi WebKittens!

Recently, I started naming threads in Linux. And I also started naming
threads created by WTF::AutomaticThread.
Previously, all the thread is not named in Linux. It makes difficult to
find problematic threads when using GDB in Linux.
For example, if you run the JSC shell, all the threads are named as "jsc"
(this is the name of the process).

The problem raised here is that we have no consistent policy for thread
names.
I picked several names in WebKit.

In WebCore,
    IDB server is "IndexedDatabase Server"
    AsyncAudioDecoder is "Audio Decoder"
    GCController is "WebCore: GCController"
    Icon Database is "WebCore: IconDatabase"
    Audio's ReverbConvolver is "convolution background thread"
The thread name used by WorkQueue in WebCore is,
    Crypto Queue is "com.apple.WebKit.CryptoQueue"
    Image decoder is "org.webkit.ImageDecoder"
    Blob utility is "org.webkit.BlobUtility"
    Data URL decoder is "org.webkit.DataURLDecoder"
In JSC
    Before this patch, all the AutomaticThreads (including JIT worklist /
DFG worklist) is "WTF::AutomaticThread"
    Super Sampler thread is "JSC Super Sampler"
    Asychronous Disasm is "Asynchronous Disassembler"
    Sampling profiler is "jsc.sampling-profiler.thread"
    WASM compiler thread is "jsc.wasm-b3-compilation.thread"
In WebKit2
    Network Cache is "IOChannel::readSync"
    IPC workqueue is "com.apple.IPC.ReceiveQueue"

To choose the appropriate naming policy, there are two requirements.
This is discussed in https://bugs.webkit.org/show_bug.cgi?id=166678 and
https://bugs.webkit.org/show_bug.cgi?id=166684

1. We should have super descriptive name including the iformation "This
thread is related to WebKit".
If we use WebKit as the framework, WebKit will launch several threads along
with the user application's threads.
So if the thread name does not include the above information, it is quite
confusing: Is this crash related to WebKit OR user's applications?
This should be met at least in macOS port. In the Linux port, this
requirement is a bit difficult to be met due to the second requirement.

2. The thread name should be <= 15 characters in Linux. <= 31 characters in
Windows.
This is super unfortunate. But we need this requirement. But in macOS, I
think we do not have any limitation like that (correct?)
I cannot find "PTHREAD_MAX_NAMELEN_NP" definition in macOS.

To meet the above requirements as much as possible, I suggest the name, "
org.webkit.MODULE.NAME(15characters)".
This policy is derived from the WorkQueue's naming policy, like
"org.webkit.ImageDecoder".
For example, we will name DFG compiler worklist thread as
"org.webkit.jsc.DFGCompiler" or "org.webkit.JavaScriptCore.DFGCompiler".

In Linux / Windows, we have the system to normalize the above name to
"NAME(15characters)".
For example, when you specify "org.webkit.jsc.DFGCompiler", it will be
shown as "DFGCompiler" in Linux.
This naming policy meets (1) in macOS. And (2) in all the environments. In
macOS, the name is not modified.
So we can see the full name "org.webkit.jsc.DFGCompiler".
In Linux, we can see "DFGCompiler", it is quite useful compared to nameless
threads.

What do you think of?

Best regards,
Yusuke Suzuki
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