Combined mode quirk removal kills performance

Combined mode quirk removal kills performance

[Frank Sorenson]

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The patch to "remove combined mode quirk" (git bisect says
8cdfb29c0cd8018f92214c11c631d8926f4cb032) makes my laptop run slower
than a dead sloth.  "hdparm -T" indicates that buffered disk reads on my
hard drive drop from 48-50 MB/sec to 1-2MB/sec, and the system is nearly
unusable.

System is a Dell Inspiron E1705 (Core 2 Duo 2.16GHz) running x86_64 FC6.

Frank
- --
Frank Sorenson - KD7TZK
Linux Systems Engineer, DSS Engineering, UBS AG
frank@tuxrocks.com
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Re: Combined mode quirk removal kills performance

[Stephen Clark]

Frank Sorenson wrote:

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>Hash: SHA1
>
>The patch to "remove combined mode quirk" (git bisect says
>8cdfb29c0cd8018f92214c11c631d8926f4cb032) makes my laptop run slower
>than a dead sloth.  "hdparm -T" indicates that buffered disk reads on my
>hard drive drop from 48-50 MB/sec to 1-2MB/sec, and the system is nearly
>unusable.
>
>System is a Dell Inspiron E1705 (Core 2 Duo 2.16GHz) running x86_64 FC6.
>
>Frank
>- --
>Frank Sorenson - KD7TZK
>Linux Systems Engineer, DSS Engineering, UBS AG
>frank@tuxrocks.com
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>Version: GnuPG v1.4.7 (GNU/Linux)
>Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org
>
>iD8DBQFGXt+JaI0dwg4A47wRAqJmAJ4k4E9ekkitOdQ72HUXbky11nD1cACghGu6
>y7Y71PPaKHYcQhYSfL55bNk=
>=MPYY
>-----END PGP SIGNATURE-----
>-
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Hi Frank,

I had the same problem. I am running FC6 and had to build a custom 
kernel that did not configure
in the old ide driver - then my speed of my harddrive went back to normal.

HTH,
Steve

-- 

"They that give up essential liberty to obtain temporary safety, 
deserve neither liberty nor safety."  (Ben Franklin)

"The course of history shows that as a government grows, liberty 
decreases."  (Thomas Jefferson)

Re: Combined mode quirk removal kills performance

[Jeff Garzik]

Andrew Morton wrote:
> So I revisited [...]
> http://bugzilla.kernel.org/show_bug.cgi?id=8636
> 
> I don't see why it's not-a-bug.  The second guy (Stephen Clark) had to toss
> out the FC6 kernel and build his own kernel to fix this regression.

(I hope you don't mind me copying linux-ide)

The combined mode removal changed the driver load configuration.  Some 
distros updated with that knowledge, but apparently Fedora did not.

Combined mode would explicitly reserve ports for libata in pci/quirks.c, 
a horrible layering violation that created special case module load 
order dependencies.

The change made libata and the old-IDE driver behave like normal Linux 
drivers.  They can both grab an entire PCI device and drive it 
correctly, so -- just like every other situation where two drivers can 
match the same PCI ID -- the one that loads first wins.

Kernel configs need to be tweaked a bit, due to this change.  That's why 
it was communicated in advance (but poorly, as it appears from your 
questions and existing bug reports).  Two common results appear in bug 
reports:

1) One possible result of a complete lack of kconfig tweaking (blindly 
hitting <enter> during 'make oldconfig') is falling back to the 
most-compatible configuration, the legacy IDE driver (either libata or 
old-IDE), with resultant slow performance.

2) "no root! panic!" and similar I-cant-find-your-hard-drive results for 
people with hardcoding root= configurations, for the minority where a 
device moved from /dev/hdX -> /dev/sdX, or vice versa.

The damage is hoped to be limited to:

* Intel ICH5/6[/7?] users with combined mode enabled, which is a 
not-small subset of all ICH[567] users.

* Users that did /not/ choose the combined_mode=libata kernel command 
line option, a popular option for restoring performance /broken/ by 
running two drivers in tandem [i.e. the old way, recently removed].

* In the combined mode configuration, one device is /dev/hdX (often the 
CD-ROM) and one device is /dev/sdX, so only one of those devices will 
move.  Standard LVM and mount-by-label/uuid found in standard distro 
installs makes this move seamless for many.

* ...in distros that hopefully took stock of their compatibility 
picture, and modified their kernel configs and drivers accordingly. 
Some distros were defaulting to combined_mode=libata (==libata drives 
all applicable Intel ICHx IDE devices), others were not.  That affects 
the decision about kernel config changes.

	Jeff