** Also affects: linux (Ubuntu Bionic)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1852257
Title:
s390/dasd: reduce the default queue depth and nr of hardware queues
Status in Ubuntu on IBM z Systems:
Confirmed
Status in linux package in Ubuntu:
New
Status in linux source package in Bionic:
New
Bug description:
SRU Justification:
==================
[Impact]
* On s390x systems with a small memory footprint, but large amounts of
DASD disks,
* the memory can get depleted (even during installation) which can
eventually lead to a situation where the OOM kicks in (followed by
even more problems).
* Starting with kernel 4.18 the patch below leads to 90% memory
consumption savings per active DASD device.
* The below backport is needed to fix this and get the improvement
into bionic's kernel 4.15.
[Fix]
* 3284da34a87ab7a527a593f89bbdaf6debe9e713 3284da3 "s390/dasd: reduce
the default queue depth and nr of hardware queues"
* Backport:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1852257/+attachment/5304830/+files/0001-s390
-dasd-reduce-the-default-queue-depth-and-nr-of-h.patch
[Test Case]
* Configure a s390x system (z/VM guest or LPAR) with only a bit RAM,
but lot's of DASDs devices.
* Now gradually enable more and more DASDs and monitor the memory
usage (be sure to exclude shared memory and cache).
* One can notice a difference in mem usage of about 10:1 per activated
DASD comparing the current stock 4.15 kernel with a patches kernel
4.15.
* With about less than 1GB memory and 40+ DASD devices one may start
to run into an OOM situation w(o the patch.
[Regression Potential]
* The regression potential can be considered as moderate, since:
* this is purely s390x specific
* it again only affects DASD disk storage (no zFCP/SCSI disk storage)
* and it's again only limited to smaller systems (so more z/VM guests
rather than LPARs).
[Other Info]
* A cherry-pick to 4.15 wasn't clean (problem in one line), hence the
backport, which applied, compiled and worked fine.
__________
This fix is already available with Ubuntu 18.10, but need also be
integrated into Ubuntu 18.04 LTS kernel (4.15).
This is available git-commit:
https://github.com/torvalds/linux/commit/3284da34a87a
Backport information will be provided.
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