Public bug reported:

As I reported on
  https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg05001.html

TL;DR:
common Host size in CI's is 1-2G - for example Ubuntu Autopkgtests 1.5G
Some of them run Guests of 0.5-1G size in TCG mode (as they often can't rely on 
having KVM available).

Due to
https://git.qemu.org/?p=qemu.git;a=commit;h=600e17b261555c56a048781b8dd5ba3985650013
in qmeu 5.0 the TB default size bumped from 32M to 1G.

The 1G TB buffer + 0.5G actual guest size + there is no dynamic
downsizing on memory pressure (never was) makes these systems go OOM-
Killing the qemu process.

Let us try to tune back the default size a bit again while the upstream
discussion is ongoing.

Currently affects in Ubuntu Groovy autopkgtest environment:
- casper
- open-iscsi
- systemd
- ubuntu-image

** Affects: qemu (Ubuntu)
     Importance: Undecided
     Assignee: Christian Ehrhardt  (paelzer)
         Status: In Progress


** Tags: update-excuse

** Changed in: qemu (Ubuntu)
       Status: New => In Progress

** Changed in: qemu (Ubuntu)
     Assignee: (unassigned) => Christian Ehrhardt  (paelzer)

** Tags added: update-excuse

** Description changed:

  As I reported on
-   https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg05001.html
+   https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg05001.html
  
  TL;DR:
  common Host size in CI's is 1-2G - for example Ubuntu Autopkgtests 1.5G
  Some of them run Guests of 0.5-1G size in TCG mode (as they often can't rely 
on having KVM available).
  
  Due to
  
https://git.qemu.org/?p=qemu.git;a=commit;h=600e17b261555c56a048781b8dd5ba3985650013
  in qmeu 5.0 the TB default size bumped from 32M to 1G.
  
  The 1G TB buffer + 0.5G actual guest size + there is no dynamic
  downsizing on memory pressure (never was) makes these systems go OOM-
  Killing the qemu process.
  
  Let us try to tune back the default size a bit again while the upstream
  discussion is ongoing.
+ 
+ Currently affects in Ubuntu Groovy autopkgtest environment:
+ - casper
+ - open-iscsi
+ - systemd
+ - ubuntu-image

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https://bugs.launchpad.net/bugs/1887763

Title:
  new default qemu TCG sizes exceed common CI setups

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