[Qemu-devel] [Bug 1609968] Re: "cannot set up guest memory" b/c no automatic clearing of Linux' cache

[Celmor]

@dgilbert-h / Dr. David Alan Gilbert
Thanks for your answer.

b)
Mounted/used block devices:
NAME    MOUNTPOINT TYPE  FSTYPE
sda                disk  crypto_LUKS
└─Data1            crypt zfs_member
├─sdb5  /          part  ext4
└─sdb6  /boot      part  vfat
sdd                disk  crypto_LUKS
└─Data2            crypt zfs_member
ZFS file system for extra space, also ZFS is the reason I'm not running the 
latest kernel...

a)
$ free
              total        used        free      shared  buff/cache   available
Mem:       16379476     7879216     1867196      188180     6633064     3587620
Swap:             0           0           0
$ qemu.sh -m 10240 && echo success || echo failed
qemu-system-x86_64: cannot set up guest memory 'pc.ram': Cannot allocate memory
failed
$ sudo sh -c 'echo 1 > /proc/sys/vm/overcommit_memory'
$ qemu.sh -m 10240 && echo success || echo failed
success

So setting /proc/sys/vm/overcommit_memory to 1 works, so I guess I'm gonna need 
to execute
sudo sh -c 'echo 1 > /proc/sys/vm/overcommit_memory'
at start of my qemu.sh script instead of the 'drop_caches' part.
I still think the kernel should do whatever function overcommit_memory is for 
automatically, bit it seams to be the fault of the kernel of my distribution 
then, thanks for your help.

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https://bugs.launchpad.net/bugs/1609968

Title:
  "cannot set up guest memory" b/c no automatic clearing of Linux' cache

Status in QEMU:
  New

Bug description:
  Version: qemu-2.6.0-1
  Kernel: 4.4.13-1-MANJARO
  Full script (shouldn't matter though): https://pastebin.com/Hp24PWNE

  Problem:
  When host has been up and used for a while cache has been filled as much that 
guest can't be started without droping caches.

  Expected behavior:
  Qemu should be able to request as much Memory as it needs and cause Linux to 
drop cache pages if needed. A user shouldn't be required to have to come to 
this conclusion and having to drop caches to start Qemu with the required 
amount of memory.

  My fix:
  Following command (as root) required before qemu start:
  # sync && echo 3 > /proc/sys/vm/drop_caches

  Example:
  $ sudo qemu.sh -m 10240 && echo success || echo failed
  qemu-system-x86_64: cannot set up guest memory 'pc.ram': Cannot allocate 
memory
  failed
  $ free
                total        used        free      shared  buff/cache   
available
  Mem:       16379476     9126884     3462688      148480     3789904     
5123572
  Swap:             0           0           0
  $ sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches'
  $ free
                total        used        free      shared  buff/cache   
available
  Mem:       16379476     1694528    14106552      149772      578396    
14256428
  Swap:             0           0           0
  $ sudo qemu.sh -m 10240  && echo success || echo failed
  success

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