Backport request to stable of two performance related fixes for xen-blkfront (3.13 fixes to earlier trees)

[Konrad Rzeszutek Wilk]

Hey Greg

This email is in regards to backporting two patches to stable that
fall under the 'performance' rule:

 bfe11d6de1c416cea4f3f0f35f864162063ce3fa
 fbe363c476afe8ec992d3baf682670a4bd1b6ce6

I've copied Jerry - the maintainer of the Oracle's kernel. I don't have
the emails of the other distros maintainers but the bugs associated with it are:

https://bugzilla.redhat.com/show_bug.cgi?id=1096909
(RHEL7)
https://bugs.launchpad.net/ubuntu/+bug/1319003
(Ubuntu 13.10)

The following distros are affected:

(x) Ubuntu 13.04 and derivatives (3.8)
(v) Ubuntu 13.10 and derivatives (3.11), supported until 2014-07
(x) Fedora 17 (3.8 and 3.9 in updates)
(x) Fedora 18 (3.8, 3.9, 3.10, 3.11 in updates)
(v) Fedora 19 (3.9; 3.10, 3.11, 3.12 in updates; fixed with latest update to 3.13), supported until TBA
(v) Fedora 20 (3.11; 3.12 in updates; fixed with latest update to 3.13), supported until TBA
(v) RHEL 7 and derivatives (3.10), expected to be supported until about 2025
(v) openSUSE 13.1 (3.11), expected to be supported until at least 2016-08
(v) SLES 12 (3.12), expected to be supported until about 2024
(v) Mageia 3 (3.8), supported until 2014-11-19
(v) Mageia 4 (3.12), supported until 2015-08-01
(v) Oracle Enterprise Linux with Unbreakable Enterprise Kernel Release 3 (3.8), supported until TBA

Here is the analysis of the problem and what was put in the RHEL7 bug.
The Oracle bug does not exist (as I just backport them in the kernel and
send a GIT PULL to Jerry) - but if you would like I can certainly furnish
you with one (it would be identical to what is mentioned below).

If you are OK with the backport, I am volunteering Roger and Felipe to assist
in jamming^H^H^H^Hbackporting the patches into earlier kernels.

Summary:
Storage performance regression when Xen backend lacks persistent-grants support

Description of problem:
When used as a Xen guest, RHEL 7 will be slower than older releases in terms
s 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).
This has been identified and fixed in the 3.13 kernel series, 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.


How reproducible:
This is always reproducible when a RHEL 7 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) - or run it under Amazon EC2
2. Install a set of RHEL 7 guests (which uses kernel 3.10).
3. Measure aggregate storage throughput from all guests.

NOTE: The storage infrastructure (e.g. local SSDs, network-attached storage)
cannot 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 if the
backend supports persistent grants.

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