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

From: Austin S. Hemmelgarn
Date: Tue Jan 05 2016 - 13:39:47 EST

On 2016-01-04 13:41, Austin S. Hemmelgarn wrote:
On 2016-01-04 12:11, Greg KH wrote:
Please provide some "real" numbers of memory savings please before
saying that this change really does save memory. Just guessing isn't
ok.
I can probably put something together to actually test this, but it will
take a while (most of my testing scripts and VM's are targeted at
regression testing of filesystems, not memory profiling of virtual
device drivers). I doubt that it will work out to any more than 16k size
difference, but that's still a few more pages (on most systems) that
could be used for other things.

As promised, I've got numbers regarding the memory impact.

The system used for testing was a para-virtualized Xen domain running a Linux kernel built from these sources:
git://github.com/Ferroin/linux.git
using the attached base config.

The domain used for testing was given 4096 MB or RAM, 4 VCPUS, a PV NIC, 2 PV disks, and had migration restrictions disabled (nomigrate=1 in the domain configuration file).

I tested stock sources with the VT subsystem enabled, stock sources with the VT subsystem disabled, and locally modified sources with MAX_NR_CONSOLES and MAX_USER_NR_CONSOLES manually changed to 31.

The testing involved booting each configuration 8 times, and comparing the MemTotal line form /proc/meminfo. None of the tests included any userspace initialization of the VT's.

Results were 100% stable across each reboot for a given configuration.

Having the full VT subsystem built in, MemTotal showed 4097176 Kb of RAM.
The manually modified version with half the number of VT's showed 4097228 Kb or RAM.
With the entire VT subsystem compiled out, MemTotal was 4097304 Kb.

This means that not including the VT subsystem resulted in a 128k reduction in runtime footprint, and having only half the number of VT's resulted in a 52k reduction. Assuming a linear correlation between the number of VT's and the runtime footprint of the subsystem, that means the subsystem itself incurs 26k of overhead, and each VT incurs approximately 1.6k of overhead.

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