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I've been trying various terminal emulators lately, from the built-in gnome-terminal, aterm, xterm, wterm, to rxvt. The test I've been doing is in this order:

  1. Open up a tmux window with 2 panes
  2. The left pane will be an verbose-intensive task such as grep a /et/c -r or a simple time seq -f 'blah blah %g' 100000
  3. The right pane will be a vim window with syntax on, opening any file that has more than >100 lines of code.

When the left pane is printing a lot of output, the right pane seems to be very slow and unresponsive, I tried to scroll in vim but it takes 1-2 second for it to change. When I try to press CtrlC on the left pane it waits for more than 10 second before it stopped

When I do the same thing in TTY (pressing CTRL+ALT+(F[1-6])), it doesn't happen and both panes are very responsive.

I've turned of some config such as antialias fonts, turn of coloring, use default settings, and change to xmonad and openbox, but it doesn't change anything.

The result of time seq -f 'blah blah %g' 100000 is not really different among these terminals, but the responsiveness is really different especially when I'm running spitted pane tmux (or other multiplexers). FYI, I'm running all of them in a maximized mode.

I've read about frame buffered terminals but not sure how does it work and how can it be used to speed up my terminal emulator.

So my question is, what makes terminal emulator far slower than TTY? Is there any possibility to make it as fast as TTY? Maybe hardware acceleration or something?. One thing I know, my resolution in X server when running a maximized terminal emulator is 1920x1080, and when I'm running TTY it is less than that, but I'm not sure how this would affect the performance.

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sounds like there is a big buffer somewhere – Thorbjørn Ravn Andersen Jun 21 '12 at 3:15
Notice tty[1-6] are not always natually fast. On one of the machines i'm using, the text console very very slow while an xterm performs decently. – soubunmei Feb 28 at 7:44

2 Answers 2

When a GUI terminal emulator prints out a string, it has to convert the string to font codepoints, send the codepoints to a font renderer, get back a bitmap and blit that bitmap to the display via the X server.

The font renderer has to retrieve the glyphs and run them (did you know that Truetype/Opentype fonts are programs running inside a virtual machine in the font renderer?). During the process of running each glyph, an insane number of decisions are made with respect to font metrics, kerning (though monospace fonts and kerning don't mix well), Unicode compliance, and that's before we even reach the rasteriser which probably uses sub-pixel addressing. The terminal then has to take the buffer produced by the font rasteriser and blit it to the right place, taking care of pixel format conversions, alpha channels (for sub-pixel addressing), scrolling (which involves more blitting), et cetera.

In comparison, writing a string to a Virtual Terminal running in text mode (note: not a graphical console) involves writing that string to video memory. ‘Hello, World!’ involves writing 13 bytes (13 16-bit words if you want colours, too). The X font rasteriser hasn't even started its stretching exercises and knuckle cracking yet, and we're done. This is why text mode was so incredibly important in decades past. It's very fast to implement. Even scrolling is easier than you think: even on the venerable Motorola 6845-based MDA and CGA, you could scroll the screen vertically by writing a single 8-bit value to a register (could be 16... it's been too long). The screen refresh circuitry did the rest. You were essentially changing the start address of the frame buffer.

There's nothing you can do to make a graphical terminal as fast as a text mode terminal on the same computer. But take heart: there have been computers with slower text modes than the slowest graphical terminal you're ever likely to see on a modern computer. The original IBM PC was pretty bad (DOS did software scrolling, sigh). When I saw my first Minix console on an 80286, I was amazed at the speed of the (jump) scrolling. Progress is good.

Update: how to accelerate the terminal

@poige has already mentioned three in his answer, but here's my own take on them:

  • Decrease the size of the terminal. My own terminals tend to grow till they fill screens, and they get slow as they do that. I get exasperated, annoyed at graphical terminals, then I resize them and everything's better. :)
  • (@poige) Use a different terminal emulator. You can get a huge speed boost at the cost of some modern features. xterm and rxvt work really well, it has a fantastic terminal emulator. I suspect your tests may have showed they perform better than the ‘modern’ ones.
  • (@poige) Don't use scalable fonts. 1986 may call and ask for its terminals back, but you can't deny they're faster. ;)
  • (@poige) Dumb down the font rasteriser by turning off anti-aliasing/sub-pixel addressing and hinting. Most of them allow overrides in environment variables, so you don't have to do this globally. Note: pointless if you choose a bitmap font.
  • This will hurt the most: don't use (multiple panes in) tmux — run two separate terminals side by side. When tmux displays two panes, it has to use terminal directives to move the cursor around a lot. Even though modern terminal libraries are very fast and good at optimising, they're still stealing bytes from your raw terminal bandwidth. To move the cursor to an arbitrary row on a DEC VT-compatible terminal, you send ESC [ row ; col H. That's 6–10 bytes. With multiple terminals, you're segregating the work, doing away with the need for positioning, optimisation, buffering and all the other stuff curses does, and making better use of multiple CPU cores.
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+1, but the tmux thing is even more complicated than just some extra escape codes being sent. Terminals are meant to scroll the whole window, not half of it. So when tmux has to move all the text in the left pane up a line, it cant just create a new line and let the terminal move it up, it has to redraw the entire pane. – Patrick Jun 21 '12 at 12:12
Quite right! I forgot about that. Although there have been terminals that could scroll part of the screen (IIRC it was called ‘protecting’ part of the screen — it was used for forms etc), it was never particularly flexible, and probably isn't supported by modern terminal emulators. Even though you find some really bizarre obsolete directives still implemented today. – Alexios Jun 21 '12 at 12:26
Replying this after 3 years but hope someone finds this useful. I notice the lag only when I do vertical split in my vim (yes, even NERDTree) but the normal split doesn't seem to be issue at all during scrolling. – shriek Oct 10 at 20:30

Meanwhile @Alexios have pretty well described all the reasons, I can mention several things, which relatively relieve the pain:

  • use bitmap fonts (Terminal, Terminus — this is really great one),
  • turn anti-aliasing off, or consider at least not using sub-pixel rendering,
  • use KDE's terminal — konsole.
share|improve this answer
Also, and this is the painful one, decrease the size of the terminal (the OP is using a 1920x1200px window). I love big terminals, and mine tend to grow till they get almost as big as the screen, but terminal speed drops as the terminal grows. Even konsole (which I favour). – Alexios Jun 21 '12 at 8:33
konsole does lazy screen updates: instead of immediately displaying output, it waits a little time for more output, so as to "batch" updates. Which is why it performs so well, to the point of being totally responsive with the OP's stress test. – ninjalj Oct 1 '13 at 11:48
Pretty sure font rendering is cached at some point. I don't think the pixels representing the letter f get re-rendered from the vector definition every time it's copied to a pixmap. (unless it has to be rendered at a new size, or a new angle). I won't dispute the fact that there are some really nice bitmap fonts, that may be the best option just for appearance alone, if they happen to exist for the size you want. – Peter Cordes Aug 6 '14 at 8:57

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