Comparison of PAE and Non-PAE 2..4.14 (p8) in high load

Comparison of PAE and Non-PAE 2..4.14 (p8) in high load

[Willi Nüßer]

Hi,

after my first posting to lkml where we compared distributor
provided kernels vs. a plain 2.4.14-pre8 it was pointed
out that between PAE and non-PAE kernels some performance
differences might exist.


We checked this last night and here are the first results.
Again,
a) the relevant quantity dialog steps per second is a
measure for the throughput our application servers runs.
b) our application server and the corresponding database
(SAP DB) run on 4 way Dell, 1 GB at boot time enabled.

Results:
---------

2.4.7 
	2.4.14p8 PAE		2.4.14p4 non- PAE
-------------------------------------------------------------
   1.80		13.42            	15.47
   1.10		13.28            	14.76
   1.20 		14.08            	14.63
   1.26     	13.17            	15.30
   1.35		13.41            	14.51


This means that we did see a performance decrease of about
6 % compared to 2.4.14p8 nonPAE but still 2.4.14p8 is an order
of magnitude faster than 2.4.7

-- 
Best regards
	Willi

-----------------------------------
Willi Nuesser
SAP Linuxlab

Re: Comparison of PAE and Non-PAE 2..4.14 (p8) in high load

[Brian Gerst]

Willi Nüßer wrote:
> 
> Hi,
> 
> after my first posting to lkml where we compared distributor
> provided kernels vs. a plain 2.4.14-pre8 it was pointed
> out that between PAE and non-PAE kernels some performance
> differences might exist.
> 
> We checked this last night and here are the first results.
> Again,
> a) the relevant quantity dialog steps per second is a
> measure for the throughput our application servers runs.
> b) our application server and the corresponding database
> (SAP DB) run on 4 way Dell, 1 GB at boot time enabled.
> 
> Results:
> ---------
> 
> 2.4.7
>         2.4.14p8 PAE            2.4.14p4 non- PAE
> -------------------------------------------------------------
>    1.80         13.42                   15.47
>    1.10         13.28                   14.76
>    1.20                 14.08                   14.63
>    1.26         13.17                   15.30
>    1.35         13.41                   14.51
> 
> This means that we did see a performance decrease of about
> 6 % compared to 2.4.14p8 nonPAE but still 2.4.14p8 is an order
> of magnitude faster than 2.4.7

PAE mode increases the size of the page table entries to 64-bits, and
the x86 doesn't do 64-bit operations very well.  Plus it has three
levels of tables to work with instead of two.  It is the only way to
support more than 4GB of memory so it's a tradeoff between high memory
support and performance.  If you have less memory, don't run a PAE
kernel.

--

				Brian Gerst

Re: Comparison of PAE and Non-PAE 2..4.14 (p8) in high load

[Willi Nüßer]

Brian Gerst wrote:

 
>>
>>Results:
>>---------
>>
>>2.4.7
>>        2.4.14p8 PAE            2.4.14p4 non- PAE
>>-------------------------------------------------------------
>>   1.80         13.42                   15.47
>>   1.10         13.28                   14.76
>>   1.20                 14.08                   14.63
>>   1.26         13.17                   15.30
>>   1.35         13.41                   14.51
>>
>>This means that we did see a performance decrease of about
>>6 % compared to 2.4.14p8 nonPAE but still 2.4.14p8 is an order
>>of magnitude faster than 2.4.7
>>
> 
> PAE mode increases the size of the page table entries to 64-bits, and
> the x86 doesn't do 64-bit operations very well.  Plus it has three 

> levels of tables to work with instead of two.  


Yes, I know the difference. The reason for this post was not blind
curiosity but the presumption that the great performance increase of 
2.4.14p8 we reported was mainly due to the PAE enabling in the 2.4.7 and 
non-PAE-enabling 2.4.14p8 in our first test.

The tests above show that this is not the case. Even with PAE 2.4.14 is
faster by an order of magnitude.

The other info we did find interesting is the actual amount of 
difference between PAE and nonPAE. The 6% we got fit very well in some
estimates I found on lkml.

-- 
Best regards
Willi

-----------------------------------
Willi Nuesser
SAP Linuxlab