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From Dirk-Willem van Gulik <di...@webweaving.org>
Subject Re: mod_disk_cache and atimes
Date Tue, 25 Mar 2008 16:54:35 GMT

On Mar 25, 2008, at 5:06 PM, Akins, Brian wrote:

>> No - it does not; so you get that speed increase (which is very
>> noticable on a swapspace/ram-disk*).
>
> I'd like to see some numbers on that.  I just did a quick test on  
> Linux and
> saw no real improvement (testing our hacked-to-heck version).


I'll see if I can get something decent - am measuring anyways.

Meanwhile  - in your test - did most of your disk-IO get cached  
regardless ?  Using a config:

	# create with
	#
	# 	M=`mdconfig -a -s 500M`
	#	newfs /dev/$M; mount /dev/$M /mnt
	#	mkdir /mnt/_cache /local0/_cache
	#	chown www:www /mnt/_cache /local0/_cache
	#
	CacheDirLevels 5
	CacheDirLength 3
	CacheIgnoreNoLastMod on
	CacheStoreNoStore	on
	CacheStorePrivate On

	TransferLog /dev/null
	ErrorLog /dev/null

	Listen 127.0.0.1:1081
	<VirtualHost *:1081>
		CacheRoot /mnt/_cache
		CacheEnable disk /
	</VirtualHost>

	Listen 127.0.0.1:1082
	<VirtualHost *:1082>
		CacheRoot /local0/_cache
		CacheEnable disk /
	</VirtualHost>

	include "conf/tset-woollight.com-no-override"

And ensuring that there is enough traffic to 'hide' the disk - and a  
reply of a log file across about 300M of data; 10% responsible for 80%  
of the traffic; remainder spread flat (not log/log) I get about:

	Requests per second: 	5146.06 [#/sec] (mean)
versus
	Requests per second:    2031.03 [#/sec] (mean)

However - drop the disk traffic and keep the footprint to 10Mbyte and  
I get:

	Requests per second: 	5121.30 [#/sec] (mean)
versus
	Requests per second:    4837.41 [#/sec] (mean)

with an standard deviation which is soo large that the latter two are  
about the same. Which supports your point perfectly.

Though if anyone can find me some more serious hardware* - I'd love to  
do this proper; as I am struggling getting a fixed mod_memm_cache and  
mod_memcached_cache to be taxed hard enough to actually measure/ 
profile sensibly (right now I saturate network(switches), irq2cpu  
affinity of the network cards and IO - and just cannot get any  
meaningful number).

Thanks,

Dw

*: anyone - feel free to ping me privately if you have a spare 3 tier  
1Gb/s network with a perfectly full duplex
    perfectly switched network fabric, a handful of 2x2 core/ 8Gb+ /  
fast scsi/sas machines (or a very decent blade
    chassis) and a nearby linux/freebsd capable sysadmin. Have SSH key  
which travels well.


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