linux-kernel - Re: [PATCH] Add possibility to set /dev/tty number

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Date:	Mon, 4 Jan 2016 09:11:21 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>
Cc:	Pierre Paul MINGOT <mingot.pierre@...il.com>, jslaby@...e.cz,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add possibility to set /dev/tty number

On Mon, Jan 04, 2016 at 11:57:33AM -0500, Austin S. Hemmelgarn wrote:
> On 2016-01-04 10:43, Greg KH wrote:
> >On Mon, Jan 04, 2016 at 04:34:56PM +0100, Pierre Paul MINGOT wrote:
> >>Hello,
> >>
> >>In Linux there is no way to set the number of tty devices or console
> >>to create. By default the kernel create 64 /dev/tty devices. what is
> >>too much for embedded system with limited resources.
> >
> >Really?  How much memory does a vt device take up?
> On a device with a simple text mode console in 80x25, a minimum of 2000
> bytes, not including anything used for character attributes, and anything
> else needed for the display and updating of the screen (I think I worked out
> once that it comes out to about 8k).  On my laptop which has a 1920x1080
> screen, using the standard 8x16 VGA font with a framebuffer console via
> i915, I get a 200x67 terminal size, which means that just the text without
> any attributes works out to a little more than 13k.  That gets doubled just
> by adding color, and probably doubled again for the other display
> attributes.  All of this also doesn't factor in the space taken up in
> devtmpfs and sysfs by the associated files (it's not much, but it's still
> wasted space).

If the console isn't initialized by userspace, is any of that space
still really being used?  Have you tried that?

> That said, there are factors to consider other than just memory footprint:
> 1. Having 64 tty devices in /dev leads to somewhat cluttered listings (on
> most small systems I see, more than two thirds of the contents of /dev are
> tty device nodes).

Not having a cluttered /dev isn't the best reasoning here :)

> 2. Most people don't know how to switch to anything higher than about tty
> 15, a majority of people who have a graphical environment use at most 2
> VT's, and a lot of embedded systems use a fixed number of VT's that is known
> prior to full production.

Agreed, but does this actually take up memory?

> 3. There is some very poorly designed software out there (at least the
> original version of ConsoleKit, and I'd be willing to bet some third-party
> vendor software) which unconditionally starts a thread or process for each
> VT in the system.  While this software should be fixed to behave properly,
> it's infeasible for most end users to do this.

If we remove the number of devices, those "broken" userspace programs
will also break, so that implies that we should not allow this change.

Please provide some "real" numbers of memory savings please before
saying that this change really does save memory.  Just guessing isn't
ok.

thanks,

greg k-h
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