swiftlang / swiftlang/swift-corelibs-libdispatch
[SR-13841] Perf Incorrectly Assigns Dispatch Calls
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- Dominant language
- C
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Description
| Previous ID | SR-13841 |
| Radar | rdar://problem/71271956 |
| Original Reporter | xanderai (JIRA User) |
| Type | Bug |
Attachment: Download
Environment
This was run on Ubuntu 18.04 with Swift for Tensorflow 0.10:
```
$ swift --version
Swift version 5.3-dev (LLVM 55d27a5828, Swift 6a5d84ec08)
Target: x86_64-unknown-linux-gnu
$ which swift
/home/xander/swift-tensorflow-RELEASE-0.10-cuda10.2-cudnn7-ubuntu18.04/usr/bin/swift
```
I don't believe it's specific to Swift for Tensorflow.
Additional Detail from JIRA
| Votes | 0 |
| Component/s | libdispatch |
| Labels | Bug |
| Assignee | None |
| Priority | Medium |
md5: 5c2364057133aadaf7ecfa69ab055195
Issue Description:
This is based on the Swift Forums discussion here.
Find attached a screenshot of a perf flamegraph that was captured on 8 hours of a Swift server that uses Swift NIO and serial async DispatchQueues. Notice that everything above the `_dispatch_call_block_and_release` is occurring on the serial DispatchQueues. We think those call stacks are not actually executing on NIO's threads, but have been incorrectly assigned to them by perf. It seems like dispatch is missing some debug symbols, which prevents perf getting an accurate handle on how we get into `_dispatch_call_block_and_release`.
This flame graph was collected using the instructions here.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the attached perf flamegraph, the linked Penguin perf guide, and the Swift Forums discussion for context. Investigate libdispatch's debug-symbol handling around _dispatch_call_block_and_release on the stated Ubuntu and Swift for TensorFlow environment; done means explaining the incorrect thread attribution and identifying a concrete fix or reproduction.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- c
- Domain
- operating-systems, performance
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100