TSServer Reference requests taking a long time

[mjbvz]

From microsoft/vscode#31344

TypeScript Version: 2.4.2

While working in the VSCode codebase, @kieferrm noticed some major slow downs while working in a TS file. It seem the references requests against TSServer are taking up to a second.

Here are his repo steps:

1. open a VS Code workspace on master, no open editors
2. search for from 'electron' in all *.ts files
3. click on the first search result and wait until TS is properly initialized
4. click on the second search result and select ipcMain in the editor
   -> caret stops blinking, it takes several seconds until the underlying word is highlighted and occurrences are shown in the scrollbar (only happens after the reference code lenses are shown)

And the TSServer log:

https://github.com/Microsoft/vscode/files/1171339/tsserver-log.txt

[mjbvz]

Nothing stood out in the logs besides the long references requests. I believe the root case may be that we are closing and then reopening the vscode project when switching between the search results

Looking at the logs, it doesn't look like this has to do-with find-all-references. I see 11::references: elapsed time (in milliseconds) 99.1908 but 2::open: async elapsed time (in milliseconds) 7332.0351.

I can reproduce this by:

• Enable "workbench.editor.enablePreview": true, in my settings
• Search for any text
• Click on first result, hover over something. Takes a while to get info.
• Click on next result -- this closes the file of the first result and opens a new one.
• Hover again -- takes a while again.

So it looks like it is because we unload the project when the file closes.
I can also reproduce the same behavior using vscode's built-in file explorer; when I click on a new file, the old file closes, closing the project, then it has to be loaded all over again.
There is a PR at #17269 for improving our watch implementation; perhaps we could consider keeping projects open for 1 second after the last time they were closed.

[kieferrm]

@Andy-MS this would be comparable to what we do in VS Code when the last TS file closes. We keep the TS language server around for while to make sure that when a TS file is open briefly afterwards we don't have to pay the startup costs again.

[jrieken]

This is a real show stopper and makes TypeScript very hard to deal with. I can reproduce this easily by just opening files in our project. Open vscode.d.ts, open shell.ts, open workbench.ts etc. with each file the project is discarded and then the same project is loaded again

[jrieken]

@Andy-MS Just to clarify how this affects the users of VS Code: When you don't use tabs and navigate around we dispose/close documents more aggressive and you are very likely to run into this on every editor input change. So, the suggestion from @kieferrm to wait a little before throwing everything away would make a lot of sense.