Re: .img on nfs, relative on ram, consuming mass ram

[Andre Przywara <andre.przywara@amd.com>]

TOURNIER Frédéric wrote:
> Hi !
> Here's my config :
> 
> Version : qemu-kvm-0.12.5, qemu-kvm-0.12.4
> Hosts : AMD 64X2, Phenom and Core 2 duo
> OS : Slackware 64 13.0
> Kernel : 2.6.35.4 and many previous versions
> 
> I use a PXE server to boot semi-diskless (swap partitions and some local stuff) stations.
> This server also serves a read-only nfs folder, with plenty of .img on it.
> When clients connects, a relative image is created in /tmp, which is a tmpfs, so hosted in ram.
> 
> And here i go on my 2G stations :
> qemu-system-x86_64 -m 1024 -vga std -usb -usbdevice tablet -localtime -soundhw es1370 /tmp/relimg.img
> qemu-system-x86_64 -m 1024 -vga std -usb -usbdevice tablet -localtime -soundhw es1370 /dev/shm/relimg.img
> 
> I tried both. Always the same result : the ram is consumed quickly, and mass swap occurs.
Which is only natural, as tmpfs is promising to never swap. So it will 
take precedence over other RAM (that's why it is limited to half of the 
memory by default). As soon as the guest has (re)written more disk 
sectors than your free RAM can hold, the system will start to swap out 
your guest RAM (and other host applications).
So in general you should avoid putting relative disk images to tmpfs if 
your host memory is limited. As a workaround you could try to further 
limit the tmpfs max size (mount -t tmpfs -o size=512M none /dev/shm), 
but this could lead to data loss in your guest as it possibly cannot 
back the written sectors anymore.
> On a 4G system, i see kvm uses more than 1024, maybe 1200.
> And everytime a launch a program inside the vm, the amount of the host free ram (not cached) diminishes, which is weird, because it should have been reserved.
KVM uses on-demand paging like other applications. So it will not 
reserve memory for your guest (unless you use hugetlbfs's -mempath):
$ kvm -cdrom ttylinux_ser.iso -nographic -m 3072M
$ top
   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND 

  6015 user      20   0 3205m 128m 3020 S    2  2.2   0:04.94 kvm 


Regards,
Andre.

> 
> So on a 2G system, swap occurs very fast and the machine slow a lot down.
> An on a total diskless system, this leads fast to a freeze.
> 
> I have no problem if i use a relative image on disk :
> qemu-system-x86_64 -m 1024 -vga std -usb -usbdevice tablet -localtime -soundhw es1370 -drive file=/mnt/hd/sda/sda1/tmp/relimg.img,cache=none

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12