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kern/39564: wapbl performance issues with disk cache flushing

>Number:         39564
>Category:       kern
>Synopsis:       wapbl performance issues with disk cache flushing
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 16 16:15:01 +0000 2008
>Originator:     Jared D. McNeill
>Release:        NetBSD 4.99.72
System: NetBSD 4.99.72 NetBSD 4.99.72 (GENERIC) #0: Mon Sep 
15 11:17:37 EDT 2008
Architecture: x86_64
Machine: amd64

System: NetBSD 4.99.72 NetBSD 4.99.72 (GENERIC) #1: Tue Sep 
16 11:07:56 EDT 2008
Architecture: i386
Machine: i386
        Disk cache flushing can severely impact performance on a wapbl-enabled
        1. Grab qt3 and qt4 distfiles from pkgsrc:
-rw-r--r--  1 jmcneill  users  14360483 Mar 15  2007 qt-x11-free-3.3.8.tar.bz2
-rw-r--r--  1 jmcneill  users  98584925 May  2 08:03 

        2. In one terminal, extract 4.4.0 tarball in a loop
$ while true; do tar -zxvf qt-x11-opensource-src-4.4.0.tar.bz2; done

        3. In another terminal, time how long it takes to extract
           the 3.3.8 distfile:
$ time tar -zxvf qt-x11-free-3.3.8.tar.bz2

        4. Results with and without disk cache flush (patch from simonb):

flush_disk_cache=0:   18.85s real    10.12s user     2.55s system
flush_disk_cache=1:  662.49s real    10.35s user     3.13s system



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