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Steve McCoy <smccoy@greatbaysoftware.com>
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Date: Fri, 20 Jul 2012 12:00:26 GMT
From: Steve McCoy <smccoy@greatbaysoftware.com>
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Subject: kern/170021: ACPI idle causes I/O slowdown as of r209897
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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
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