Mounting host files makes docker unusably slow.

[BrendonW]

• I have tried with the latest version of Docker Desktop
• I have tried disabling enabled experimental features
• I have uploaded Diagnostics
• Diagnostics ID: 42E986E0-66D2-45E5-8349-7C213B28992A/20210416214838

Expected behavior

Prior to upgrading to 3.3.0, running a command in a container that mounted a host file system took a very similar length of time as running the same container without mounting the files.

Actual behavior

The container has a rails server in it:-

time docker run -ti --rm <container> rake --tasks
real 0m6.962s
user 0m0.115s
sys 0m0.067s

Same container mounting the identical rails source tree from a host directory:-

time docker run -ti -rm --mount type=bind,source="$(pwd)/rails",target="/home/hosting/rails" <container> rake --tasks
real 2m17.010s
user 0m0.135s
sys 0m0.068s

This makes it impossible to deploy a stack that mounts an editable source tree.

Information

• This is reproducible on demand. A coworker using an M1 mac has the exact same problem.
• During the huge time difference between the two cases, the hyperkit CPU usage stays over 300-400% in the activity monitor.
• This is a new problem
• The problem appeared when we updated to 3.3.0
• macOS Version: 11.2.3
• Intel chip or Apple chip: 3.2 GHz 6-Core Intel Core i7
• Docker Desktop Version: 3.3.1 (63152)

Steps to reproduce the behavior

1. Run a rails command in a container that mounts the rails source tree.
2. There is something about how Rails accesses the files during startup that seems to be making it behave so badly in these recent versions.
3. If needed, I'll try and create a simple container to reproduce this.

[adrubesh]

There a long discussion about this here that might help you: #3677

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