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[PR #4890] Use estimated formatted lengths to optimize performance of VT rendering #26016

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opened 2026-01-31 09:13:22 +00:00 by claunia · 0 comments
claunia commented 2026-01-31 09:13:22 +00:00
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Original Pull Request: https://github.com/microsoft/terminal/pull/4890

State: closed
Merged: Yes

Summary of the Pull Request

Allows VT engine methods that print formatted strings (cursor movements, color changes, etc.) to provide a guess at the max buffer size required eliminating the double-call for formatting in the common case.

PR Checklist

  • Found while working on #778
  • CLA signed. If not, go over here and sign the CLA
  • Tests added/passed
  • Am core contributor.

Detailed Description of the Pull Request / Additional comments

  • The most common case for VT rendering is formatting a few numbers into a sequence. For the most part, we can already tell the maximum length that the string could be based on the number of substitutions and the size of the parameters.
  • The existing formatting method would always double-call. It would first call for how long the string was going to be post-formatting, allocate that memory, then call again and fill it up. This cost two full times of running through the string to find a length we probably already knew for the most part.
  • Now if a size is provided, we allocate that first and attempt the "second pass" of formatting directly into the buffer. This saves the count step in the common case.
  • If this fails, we fall back and do the two-pass method (which theoretically means the bad case is now 3 passes.)
  • The next biggest waste of time in this method was allocating and freeing strings for every format pass. Due to the nature of the VT renderer, many things need to be formatted this way. I've now instead moved the format method to hold a static string that really only grows over the course of the session for all of these format operations. I expect a majority of the time, it will only be consuming approximately 5-15 length of a std::string of memory space. I cannot currently see a circumstance where it would use more than that, but I'm consciously trading memory usage when running as a PTY for overall runtime performance here.

Validation Steps Performed

  • Ran the thing manually and checked it out with wsl and cmatrix and Powershell and such attached to Terminal
  • Wrote and ran automated tests on formatting method
**Original Pull Request:** https://github.com/microsoft/terminal/pull/4890 **State:** closed **Merged:** Yes --- ## Summary of the Pull Request Allows VT engine methods that print formatted strings (cursor movements, color changes, etc.) to provide a guess at the max buffer size required eliminating the double-call for formatting in the common case. ## PR Checklist * [x] Found while working on #778 * [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA * [x] Tests added/passed * [x] Am core contributor. ## Detailed Description of the Pull Request / Additional comments - The most common case for VT rendering is formatting a few numbers into a sequence. For the most part, we can already tell the maximum length that the string could be based on the number of substitutions and the size of the parameters. - The existing formatting method would always double-call. It would first call for how long the string was going to be post-formatting, allocate that memory, then call again and fill it up. This cost two full times of running through the string to find a length we probably already knew for the most part. - Now if a size is provided, we allocate that first and attempt the "second pass" of formatting directly into the buffer. This saves the count step in the common case. - If this fails, we fall back and do the two-pass method (which theoretically means the bad case is now 3 passes.) - The next biggest waste of time in this method was allocating and freeing strings for every format pass. Due to the nature of the VT renderer, many things need to be formatted this way. I've now instead moved the format method to hold a static string that really only grows over the course of the session for all of these format operations. I expect a majority of the time, it will only be consuming approximately 5-15 length of a std::string of memory space. I cannot currently see a circumstance where it would use more than that, but I'm consciously trading memory usage when running as a PTY for overall runtime performance here. ## Validation Steps Performed - Ran the thing manually and checked it out with wsl and cmatrix and Powershell and such attached to Terminal - Wrote and ran automated tests on formatting method
claunia added the pull-request label 2026-01-31 09:13:22 +00:00
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Reference: starred/terminal#26016
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