Vim Slow Buffers

On Monday I wrote about how QuickFix and friends are slow. I was legitimately chasitized on reddit for giving up too soon in trying to find a solution, so I did some more digging.

This was not the first time in recent memory that I have been told that I did not properly find the root cause of a problem, so I decided to treat this like an exercize and actually find the cause. The first thing I did was to create a way to reproduce the issue and send an email to the Vim mailing list. You might want to finish this post before reading it, as it includes (some) resolutions.

Rocky Start

Here is where I made my first, critical mistake. This is how I reproduced the issue:

mkdir -p ~/.vim/ftdetect
echo 'autocmd BufNew,BufNewFile,BufRead *.md :set filetype=markdown' >  ~/.vim/ftdetect/markdown.vim
mkdir testing
cd testing
touch {1..500}.md
vim

And then in vim:

:args *

I was unclear in my email and expected the Vim maintainers to read my mind; the fast version that I did not share was to use ~/.vim/filetype.vim and put something like this in the file, which came straight from the vim documentation:

if exists("did_load_filetypes")
  finish
endif
augroup filetypedetect
  au! BufRead,BufNewFile *.md set filetype=markdown
augroup END

The documentation is a mess, in my opinion. It first shows ftdetect files, but then later in the paragraphs marked with a C says that if your file type can be detected by name you should use a filetype.vim script. I still haven’t gotten a straight answer on this one and what the right thing to do is.

Digging

First I attempted to bisect the issue by building older versions of Vim. I couldn’t bisect it because I never found a version that didn’t have the problem.

Next I profiled the actual problem. Vim has some rudimentary profiling built in and I copied some mappings to assist in such endeavours. So I profiled both versions of my code by doing the following in Vim:

\DD
:args *
\DQ

If you are interested in the profiles you can inspect them here. The problem is pretty clear though; the markdown support files are getting sourced for each buffer, instead of just once.

Bram Moolenar, the author and maintainer of Vim, suggested that I crank of the verbosity of vim (by starting it with vim -V10) and see if that helps. That might have helped except I could not figure out how to extract the information from :messages.

Next I tried to see if this was limited to markdown or if all filetypes have this problem. I tried having *.md map to both perl (an existing filetype) and sillybonk (a non existant filetype.) Neither exhibited the problem.

With this information in hand I decided I’d bisect the files that were sourced O(n) times. Because I couldn’t extract the information from :messages I just used strace to see what all files vim read. Here’s how I did that:

strace to a file, only logging open system calls, to the strace.log file:

strace -etrace=open -ostrace.log vi

Then, after reproducing the problem, I would extract the file list with counts:

$ cat strace3.log | grep -v ENOENT | grep -vF 'open("."' |
      grep -v O_DIRECTORY | cut -d'"' -f2 |
      sort | uniq -c | sort -n

    502 /usr/share/vim/vim80/syntax/vb.vim
    502 /usr/share/vim/vim80/syntax/markdown.vim
    502 /usr/share/vim/vim80/syntax/javascript.vim
    502 /usr/share/vim/vim80/syntax/html.vim
    502 /usr/share/vim/vim80/syntax/css.vim
    502 /usr/share/vim/vim80/ftplugin/markdown.vim
    502 /usr/share/vim/vim80/ftplugin/html.vim
    502 /home/frew/code/dotfiles/vim/bundle/splitjoin/ftplugin/html/splitjoin.vim
      7 /usr/share/vim/vim80/filetype.vim
      6 /home/frew/.cache/ctrlp/mru/cache.txt
      5 /usr/share/vim/vim80/syntax/syncolor.vim
      4 /home/frew/code/dotfiles/vim/ftdetect/tt.vim
    ...

A coworker thought it was weird that html, css, javascript, and vb were being loaded, but I assumed it was because markdown is typically implemented as an overlay on html. I checked and I was correct..

So with this information in hand I decided I’d try to figure out which of the eight files above was causing issues. My first step was vb, which is loaded from html, I think because IE supports (supported?) a weird thing called VBScript when they wanted their own variant of JavaScript.

I tried deleting and emptying the syntax/vb.vim file but got errors, so after some dinking around with it I replaced the contents with

let b:current_syntax = "vb"

And the problem went away.

Wasting Time

I sent an update to the Vim Mailing List about this and I was mostly brushed aside as not knowing what I was talking about. I reproduced the problem in a fresh environment, and decided to let it be at that point because I had a solution and I could live with it.

Shortly after I sent my update Antony Scriven replied to my email pointing out that my ftdetect hook was using BufNew and probably shouldn’t be. Frustratingly, when I’d made my filetype.vim I’d left that out. So that explained my first mystery, why ftdetect and filetype.vim acted differently: because I put different stuff in them.

I never should have had BufNew in the autocmd.

The problem, Antony explained, is that when you load, for example, 500 files, using :args * (or many other similar commands) is that you allocate 500 buffers, and each of those allocations triggers all of the BufNew autocommands immediately, where BufRead won’t happen to you actually load that buffer. To see it for yourself, set up a test like I did in my rocky start but before you do :args * run

:autocmd BufNew  *.md echo 'markdown  new'
:autocmd BufRead *.md echo 'markdown read'

Overall this was frustrating to me. I have no idea where the BufNew came from in my command (though it looks like I am not alone,) but at least removing BufNew speeds up things that should not be slow. Interestingly, I tried this with NeoVim, and while it still had the speed issue, it was significantly reduced. From 19s to 5s, to be precise. That kind of improvement alone might make me consider using it for terminal editing.

I hope at the very least some of the debugging examples above are helpful. Clearly they did not help me, but they may have sped up the process some.


If you’d like to learn more about vim, I can recommend two excellent books. I first learned how to use vi from Learning the vi and Vim Editors. The new edition has a lot more information and spends more time on Vim specific features. It was helpful for me at the time, and the fundamental model of vi is still well supported in Vim and this book explores that well.

Second, if you really want to up your editing game, check out Practical Vim. It’s a very approachable book that unpacks some of the lesser used features in ways that will be clearly and immediately useful. I periodically review this book because it’s such a treasure trove of clear hints and tips.

Posted Fri, May 26, 2017