I sent an SRU request to the Ubuntu kernel team mailing list. However,
since Saucy will soon be EOL, we are only accepting critical CVE
patches.
It might be best to upgrade to Trusty, which should already have this
fix.
** Changed in: linux (Ubuntu Saucy)
Status: In Progress => Won't Fix
** Changed in: linux (Ubuntu)
Status: In Progress => Won't Fix
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https://bugs.launchpad.net/bugs/1319003
Title:
Storage performance regression when Xen backend lacks persistent-
grants support
Status in “linux” package in Ubuntu:
Won't Fix
Status in “linux” source package in Saucy:
Won't Fix
Bug description:
Description of problem:
When used as a Xen guest, Ubuntu 13.10 may be slower than older releases in
terms of storage performance. This is due to the persistent-grants feature
introduced in xen-blkfront on the Linux Kernel 3.8 series. From 3.8 to 3.12
(inclusive), xen-blkfront will add an extra set of memcpy() operations
regardless of persistent-grants support in the backend (i.e. xen-blkback, qemu,
tapdisk). Many Xen dom0s do not have backend persistent-grants support (such as
Citrix XenServer and any Linux distro with Kernel prior to 3.8). This has been
identified and fixed in the 3.13 kernel series [1], but was not backported to
previous LTS kernels due to the nature of the bug (performance only).
While persistent grants reduce the stress on the Xen grant table and
allow for much better aggregate throughput (at the cost of an extra
set of memcpy() operations), adding the copy overhead when the feature
is unsupported on the backend combines the worst of both worlds.
This is particularly noticeable when intensive storage workloads are
active from many guests.
The graphs attached show storage throughput numbers for Linux guests
using kernel 3.12.9 (Graph 1) and 3.13.7 (Graph 2) running on a Citrix
XenServer development build. The server had 4 storage repositories
(SRs) with 1 Micron P320 SSD per SR (i.e. 10 VMs per SR means 40 VMs
in total). When using 3.12.9 kernel, the regression is clearly visible
for more than 2 VMs per SR and block sizes larger than 64 KiB. The
workload consisted of sequential reads on pre-allocated raw LVM
logical volumes.
[1] Commits by Roger Pau Monné:
bfe11d6de1c416cea4f3f0f35f864162063ce3fa
fbe363c476afe8ec992d3baf682670a4bd1b6ce6
Version-Release number of selected component (if applicable):
xen-blkfront of Linux kernel 3.11
How reproducible:
This is always reproducible when a Ubuntu 13.10 guest is running on Xen and
the storage backend (i.e. xen-blkback, qemu, tapdisk) does not have support for
persistent grants.
Steps to Reproduce:
1. Install a Xen dom0 running a kernel prior to 3.8 (without
persistent-grants support).
2. Install a set of Ubuntu 13.10 guests (which uses kernel 3.11).
3. Measure aggregate storage throughput from all guests.
NOTE: The storage infrastructure (e.g. local SSDs, network-attached
storage) should not be a bottleneck in itself. If tested on a single
SATA disk, for example, the issue will probably be unnoticeable as the
infrastructure will be limiting response time and throughput.
Actual results:
Aggregate storage throughput will be lower than with a xen-blkfront versions
prior to 3.8 or newer than 3.12.
Expected results:
Aggregate storage throughput should be at least as good or better than
previous (or newer) versions of Ubuntu in cases where the backend doesn't
support persistent grants.
Additional info:
Given that this is fixed on newer kernels, we urge that a backport of the
relevant patches to the 3.11 stable branch is requested. According to the rules
in: https://www.kernel.org/doc/Documentation/stable_kernel_rules.txt, the
patches would be accepted on the grounds of:
- Serious issues as reported by a user of a distribution kernel may also
be considered if they fix a notable performance or interactivity issue.
As these fixes are not as obvious and have a higher risk of a subtle
regression they should only be submitted by a distribution kernel
maintainer and include an addendum linking to a bugzilla entry if it
exists and additional information on the user-visible impact.
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