(I don't seem to be talking about Zram here, but bear with me for a few
paragraphs. And if someone already has the answers on the Flash
investigation, I'm all ears.)
With LTSP running on a modest test server running Lubuntu Raring
(13.04), I started trying to determine what minimum client configuration
(CPU, RAM, GPU/VPU, VRAM) would play YouTube Flash videos nicely enough
in a default player window size, assuming no bottleneck on the server or
the network. (I know, Flash is not the future, but today it is still a
big player.)
I aimed to start with a higher-spec client and then start working down
from there on the various components. I started by varying just the RAM.
Config #1: Pentium D 830 @ 3.0 GHz per core, *1 GB RAM*, PCI Express x16
graphics with ATI Radeon 1600PRO VPU, 256 MB VRAM
Result: OK
Config #2: Pentium D 830 @ 3.0 GHz per core, *512 MB RAM*, PCI Express
x16 graphics with ATI Radeon 1600PRO VPU, 256 MB VRAM
Result: OK
Config #3: Pentium D 830 @ 3.0 GHz per core, *256 MB RAM*, PCI Express
x16 graphics with ATI Radeon 1600PRO VPU, 256 MB VRAM
Result: Audio/video out of sync, some jerking, some horizontal lines
But in config #3, I also found that on a couple occasions, the *server*
hung, especially when I was starting/stopping Firefox on the client. I
forgot about trying the Magic SysRq keys and resorted to hard shutdowns.
So the Flash performance testing seemed to have become something more
serious, in which an unhappy/inadequate client could bring down the server.
Looking for a memory-related problem, I arrived at some troubling
observations about zram (for some info about using zram for swap, see
http://www.webupd8.org/2011/10/increased-performance-in-linux-with.html).
Output from 'sudo parted -l' includes:
Error: /dev/zram0: unrecognised disk label
And syslog messages on server and client show a slew of errors:
Lubuntu kernel: Buffer I/O error on device zram0, logical block 128247
Syslog also indicates that half of memory was given to zram to form its
block swap device.
So I wonder: Is half of memory dedicated to something that isn't
working? Will machines hang when swap is needed?
More research found that zram has been integrated in the mainline kernel
for some time, but that it is generally not configured to be used by
default. However, the Ubuntu roadmap calls for it to be part of the
default configuration in the next release, 13.10 Saucy. Currently
though, there is a high-priority scramble because it has come to light
that zram suffers from some serious bugs.
So why is zram causing a problem on my 13.04 setup? Because ltsp-client
Recommends zram-config, which is the package that configures zram for
swap usage. Zram is then configured for usage on the client and also
the server (at least with LTSP-PNP).
Maybe zram will be fixed with haste for 13.10 (due in mid-October), and
maybe a fix will be backported with haste to 13.04, but if not I'm
wondering what can/should be done. If I turn off zram on server and/or
client, what would the expected consequences be?
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