Bug 257301

Summary:	RHEL 5 is 20% slower than RHEL 4.x on same hardware
Product:	Red Hat Enterprise Linux 5	Reporter:	Simon Gao <gao>
Component:	gcc	Assignee:	Jakub Jelinek <jakub>
Status:	CLOSED WONTFIX	QA Contact:
Severity:	high	Docs Contact:
Priority:	medium
Version:	5.0	CC:	dzickus
Target Milestone:	---
Target Release:	---
Hardware:	x86_64
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	Bug Fix
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2007-09-14 09:32:11 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description Simon Gao 2007-08-27 18:23:23 UTC

Description of problem:

On the identical hardware, RHEL 5 Xen-enabled kernel is 20% or more slower than
RHEL 4.x

Version-Release number of selected component (if applicable):

2.6.18-8.1.8.el5xen

How reproducible:

Consistently. On HP dx5150 or Dell Precision 390. 

Steps to Reproduce:
1. Use two identical machines or load RHEL4 run tests first and then reload 
   with RHEL 5 and run the same tests as below
2. Untar kernel-2.6.22.5 to a scratch directory on local disk
3. Run "cd /tmp/linux-2.6.22.5; make oldconfig", then run "time make".
  
Actual results:

RHEL5 takes 20% or longer to finish compiling the same kernel than RHEL4.

Expected results:

RHEL5 takes same amount of time or less to compile linux kernel as RHEL4.

Additional info:

Comment 1 Simon Gao 2007-08-27 20:49:19 UTC

Here are some test results:

$ time tar -zxvf linux-2.6.22.5.tar.gz

RHEL5

Non-xen kernel

real    0m4.330s
user    0m3.226s
sys     0m2.932s

Xen kernel

real    0m4.009s
user    0m2.448s
sys     0m2.248s


RHEL4

real    0m6.685s
user    0m2.657s
sys     0m3.516s


$ time make

RHEL5

Non-xen kernel

real    28m5.217s
user    25m47.617s
sys     4m0.718s

Xen kernel

real    30m2.323s
user    24m14.195s
sys     5m25.640s


RHEL4

real    18m44.957s
user    16m17.779s
sys     2m17.887s

Comment 2 Petr Machata 2007-08-29 15:38:11 UTC

You probably wanted to select a component "kernel".

Comment 3 Stephen Tweedie 2007-09-13 14:32:13 UTC

RHEL5
Non-xen kernel
real    28m5.217s

Xen kernel
real    30m2.323s

RHEL4
real    18m44.957s

So, RHEL-5 is actually something like 50% slower here.

But...  this does not look like a kernel issue.  It's certainly not a Xen issue,
as the difference between the RHEL-5 xen and non-xen kernels is tiny compared to
the difference between RHEL-4 and RHEL-5.

But RHEL-5 has a newer compiler that is doing more, and different,
optimisations; it is entirely reasonable that it may take longer for some jobs.
 Indeed, the numbers above show that the bulk of the difference is accounted for
in user time, ie. time spent in the compiler, not the kernel.  

We cannot consider this to be a kernel issue unless it is the same compiler
version being tested in each case.

Comment 5 Jakub Jelinek 2007-09-14 09:32:11 UTC

RHEL4 has gcc 3.4, which had only RTL optimization passes, gcc 4.1 has both
tree-SSA and RTL optimization passes, so roughly 100 new optimization passes.
While a lot of work has been done to improve compile speed and it is still
ongoing (e.g. GCC 4.3 will have some compile time speed improvements), some
slowdown from 3.4.x is to be expected.  You get for that price better optimized
code in many cases and also some new GCC extensions that aren't really doable
on the RTL representation (e.g. -D_FORTIFY_SOURCE with gcc 3.4.x is orders
of magnitude worse protection than in gcc 4.0.x+).