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Date:      Fri, 20 Jul 2012 12:00:26 GMT
From:      Steve McCoy <smccoy@greatbaysoftware.com>
To:        freebsd-gnats-submit@FreeBSD.org
Subject:   kern/170021: ACPI idle causes I/O slowdown as of r209897
Message-ID:  <201207201200.q6KC0QaW058779@red.freebsd.org>
Resent-Message-ID: <201207201210.q6KCAAmU052742@freefall.freebsd.org>

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>Number:         170021
>Category:       kern
>Synopsis:       ACPI idle causes I/O slowdown as of r209897
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Fri Jul 20 12:10:10 UTC 2012
>Closed-Date:
>Last-Modified:
>Originator:     Steve McCoy
>Release:        8.3-STABLE
>Organization:
>Environment:
FreeBSD 4oneQA1 8.1-PRERELEASE FreeBSD 8.1-PRERELEASE #6 r209897: Tue Jul 17 09:42:40 EDT 2012     smccoy@newcastle83:/usr/obj/usr/relbuild/os/8revs/testk/sys/GENERIC  i386
>Description:
We saw this when moving from 8.1 to 8.3; I later found that r209897 is the specific change that introduces this issue. Kernels after r209897 show a 20% drop in performance for Postgres's pgbench, and we've also been using "bonnie", which shows similar or worse drops.

We have a possible workaround: Eric van Gyzen on freebsd-stable suggested changing the value of the machdep.idle sysctl from "acpi". I've tried "mwait" so far and it restores I/O performance (or bettered it in a couple cases).

Our system is an Intel S5520UR Server with 12 GB RAM, dual 4-core CPUs. Storage is a LSI MegaSAS 1078 controller (mfi) in a RAID-10 configuration, using UFS + geom_journal for filesystem. 

Here's a summary of the averaged bonnie outputs for r209896, r209897, and r209897 with machdep.idle=mwait:

              ------------Sequential Output------------ ------Sequential Input----- ----Random---
              --Per Char-- -----Block---- ---Rewrite--  --Per Char-- -----Block---- ----Seeks----
Machine    MB  K/sec  %CPU  K/sec    %CPU  K/sec  %CPU  K/sec   %CPU   K/sec   %CPU   /sec   %CPU

209896    100 141372 99.66 176170   54.72 226918 62.42 153879 100.04 3551889  98.74 275536 232.08
209897    100  99306 99.78  89651.8 49.82 110016 74.5  152890  99.52 4443177  95.72 171110 181.42
 +mwait   100 177231 99.98 364950   66.14 423017 66.2  153827  99.26 3288740 103.1  279108 240.38

The send-pr web page indicates the formatting of the above may get messed up, so here's an alternative view, if you can't read it: http://itsapad.appspot.com/262002/raw
>How-To-Repeat:

>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted:



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