Bug 10810

Summary: Performance regression on DAC960 and kernel 2.6.24+
Product: IO/Storage Reporter: Alessandro Polverini (alex)
Component: Block LayerAssignee: Jens Axboe (axboe)
Status: REJECTED INVALID    
Severity: high CC: bunk
Priority: P1    
Hardware: All   
OS: Linux   
Kernel Version: 2.6.24, 2.6.25 Subsystem:
Regression: Yes Bisected commit-id:

Description Alessandro Polverini 2008-05-28 03:52:37 UTC 
Latest working kernel version:
2.6.23

Earliest failing kernel version:
2.6.24

Distribution:
Debian

Hardware Environment:
00:00.0 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
00:00.1 RAM memory: nVidia Corporation C51 Memory Controller 0 (rev a2)
00:00.2 RAM memory: nVidia Corporation C51 Memory Controller 1 (rev a2)
00:00.3 RAM memory: nVidia Corporation C51 Memory Controller 5 (rev a2)
00:00.4 RAM memory: nVidia Corporation C51 Memory Controller 4 (rev a2)
00:00.5 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
00:00.6 RAM memory: nVidia Corporation C51 Memory Controller 3 (rev a2)
00:00.7 RAM memory: nVidia Corporation C51 Memory Controller 2 (rev a2)
00:02.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
00:03.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
00:04.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
00:05.0 VGA compatible controller: nVidia Corporation C51G [GeForce 6100] (rev a2)
00:09.0 RAM memory: nVidia Corporation MCP51 Host Bridge (rev a2)
00:0a.0 ISA bridge: nVidia Corporation MCP51 LPC Bridge (rev a2)
00:0a.1 SMBus: nVidia Corporation MCP51 SMBus (rev a2)
00:0b.0 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
00:0b.1 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
00:0d.0 IDE interface: nVidia Corporation MCP51 IDE (rev a1)
00:0e.0 IDE interface: nVidia Corporation MCP51 Serial ATA Controller (rev a1)
00:10.0 PCI bridge: nVidia Corporation MCP51 PCI Bridge (rev a2)
00:14.0 Bridge: nVidia Corporation MCP51 Ethernet Controller (rev a1)
00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] HyperTransport Technology Configuration
00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Address Map
00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] DRAM Controller
00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Miscellaneous Control
04:08.0 RAID bus controller: Mylex Corporation AcceleRAID 352/170/160 support Device (rev 02)

Software Environment:
Debian Lenny 64bit

Problem Description:
I/O Access is very slow on some condition, for example samba users can't write more than a few KB/sec on the shares.
Also tomcat is veeeery slow to startup (at least 3 times the normal time).

Steps to reproduce:
Simply boot with the new kernel
Comment 1 Anonymous Emailer 2008-05-28 10:58:17 UTC 
Reply-To: akpm@linux-foundation.org


(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Wed, 28 May 2008 03:52:37 -0700 (PDT) bugme-daemon@bugzilla.kernel.org wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=10810
> 
>            Summary: Performance regression on DAC960 and kernel 2.6.24+
>            Product: IO/Storage
>            Version: 2.5
>      KernelVersion: 2.6.24, 2.6.25
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: high
>           Priority: P1
>          Component: Block Layer
>         AssignedTo: axboe@kernel.dk
>         ReportedBy: alex@nibbles.it
> 
> 
> Latest working kernel version:
> 2.6.23
> 
> Earliest failing kernel version:
> 2.6.24
> 
> Distribution:
> Debian
> 
> Hardware Environment:
> 00:00.0 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
> 00:00.1 RAM memory: nVidia Corporation C51 Memory Controller 0 (rev a2)
> 00:00.2 RAM memory: nVidia Corporation C51 Memory Controller 1 (rev a2)
> 00:00.3 RAM memory: nVidia Corporation C51 Memory Controller 5 (rev a2)
> 00:00.4 RAM memory: nVidia Corporation C51 Memory Controller 4 (rev a2)
> 00:00.5 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
> 00:00.6 RAM memory: nVidia Corporation C51 Memory Controller 3 (rev a2)
> 00:00.7 RAM memory: nVidia Corporation C51 Memory Controller 2 (rev a2)
> 00:02.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> 00:03.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> 00:04.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> 00:05.0 VGA compatible controller: nVidia Corporation C51G [GeForce 6100]
> (rev
> a2)
> 00:09.0 RAM memory: nVidia Corporation MCP51 Host Bridge (rev a2)
> 00:0a.0 ISA bridge: nVidia Corporation MCP51 LPC Bridge (rev a2)
> 00:0a.1 SMBus: nVidia Corporation MCP51 SMBus (rev a2)
> 00:0b.0 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
> 00:0b.1 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
> 00:0d.0 IDE interface: nVidia Corporation MCP51 IDE (rev a1)
> 00:0e.0 IDE interface: nVidia Corporation MCP51 Serial ATA Controller (rev
> a1)
> 00:10.0 PCI bridge: nVidia Corporation MCP51 PCI Bridge (rev a2)
> 00:14.0 Bridge: nVidia Corporation MCP51 Ethernet Controller (rev a1)
> 00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> HyperTransport Technology Configuration
> 00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> Address
> Map
> 00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] DRAM
> Controller
> 00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> Miscellaneous Control
> 04:08.0 RAID bus controller: Mylex Corporation AcceleRAID 352/170/160 support
> Device (rev 02)
> 
> Software Environment:
> Debian Lenny 64bit
> 
> Problem Description:
> I/O Access is very slow on some condition, for example samba users can't
> write
> more than a few KB/sec on the shares.
> Also tomcat is veeeery slow to startup (at least 3 times the normal time).
> 
> Steps to reproduce:
> Simply boot with the new kernel

Oh dear.

There's been only one change to DAC960.c in that timeframe:

commit 0156c2547e92df559d5592aad9535838ef459615
Author: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Date:   Tue Dec 11 17:43:15 2007 -0500

    blk_end_request: changing DAC960 (take 4)
    
    This patch converts DAC960 to use blk_end_request interfaces.
    Related 'UpToDate' arguments are converted to 'Error'.
    
    Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
    Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
    Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

:100644 100644 9030c37... cd03473... M  drivers/block/DAC960.c

commit 117636092a87a28a013a4acb5de5492645ed620f
Author: Ralf Baechle <ralf@linux-mips.org>
Date:   Tue Oct 23 20:42:11 2007 +0200

    [PATCH] Fix breakage after SG cleanups

and I don't see how it could cause this.  The breakage is probably
external to the driver.

I don't know what it could be and I don't know anyone who can be asked
to look into it.

If you have time, the only way I can think of getting to the bottom of
this is if you were to run a git bisection search as per
http://www.kernel.org/doc/local/git-quick.html.  Sorry.
Comment 2 Anonymous Emailer 2008-05-28 11:34:53 UTC 
Reply-To: James.Bottomley@HansenPartnership.com

On Wed, 2008-05-28 at 10:58 -0700, Andrew Morton wrote:
> On Wed, 28 May 2008 03:52:37 -0700 (PDT) bugme-daemon@bugzilla.kernel.org
> wrote:
> 
> > http://bugzilla.kernel.org/show_bug.cgi?id=10810
> > 
> >            Summary: Performance regression on DAC960 and kernel 2.6.24+
> >            Product: IO/Storage
> >            Version: 2.5
> >      KernelVersion: 2.6.24, 2.6.25
> >           Platform: All
> >         OS/Version: Linux
> >               Tree: Mainline
> >             Status: NEW
> >           Severity: high
> >           Priority: P1
> >          Component: Block Layer
> >         AssignedTo: axboe@kernel.dk
> >         ReportedBy: alex@nibbles.it
> > 
> > 
> > Latest working kernel version:
> > 2.6.23
> > 
> > Earliest failing kernel version:
> > 2.6.24
> > 
> > Distribution:
> > Debian
> > 
> > Hardware Environment:
> > 00:00.0 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
> > 00:00.1 RAM memory: nVidia Corporation C51 Memory Controller 0 (rev a2)
> > 00:00.2 RAM memory: nVidia Corporation C51 Memory Controller 1 (rev a2)
> > 00:00.3 RAM memory: nVidia Corporation C51 Memory Controller 5 (rev a2)
> > 00:00.4 RAM memory: nVidia Corporation C51 Memory Controller 4 (rev a2)
> > 00:00.5 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
> > 00:00.6 RAM memory: nVidia Corporation C51 Memory Controller 3 (rev a2)
> > 00:00.7 RAM memory: nVidia Corporation C51 Memory Controller 2 (rev a2)
> > 00:02.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> > 00:03.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> > 00:04.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> > 00:05.0 VGA compatible controller: nVidia Corporation C51G [GeForce 6100]
> (rev
> > a2)
> > 00:09.0 RAM memory: nVidia Corporation MCP51 Host Bridge (rev a2)
> > 00:0a.0 ISA bridge: nVidia Corporation MCP51 LPC Bridge (rev a2)
> > 00:0a.1 SMBus: nVidia Corporation MCP51 SMBus (rev a2)
> > 00:0b.0 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
> > 00:0b.1 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
> > 00:0d.0 IDE interface: nVidia Corporation MCP51 IDE (rev a1)
> > 00:0e.0 IDE interface: nVidia Corporation MCP51 Serial ATA Controller (rev
> a1)
> > 00:10.0 PCI bridge: nVidia Corporation MCP51 PCI Bridge (rev a2)
> > 00:14.0 Bridge: nVidia Corporation MCP51 Ethernet Controller (rev a1)
> > 00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> > HyperTransport Technology Configuration
> > 00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> Address
> > Map
> > 00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> DRAM
> > Controller
> > 00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> > Miscellaneous Control
> > 04:08.0 RAID bus controller: Mylex Corporation AcceleRAID 352/170/160
> support
> > Device (rev 02)
> > 
> > Software Environment:
> > Debian Lenny 64bit
> > 
> > Problem Description:
> > I/O Access is very slow on some condition, for example samba users can't
> write
> > more than a few KB/sec on the shares.
> > Also tomcat is veeeery slow to startup (at least 3 times the normal time).
> > 
> > Steps to reproduce:
> > Simply boot with the new kernel
> 
> Oh dear.
> 
> There's been only one change to DAC960.c in that timeframe:
> 
> commit 0156c2547e92df559d5592aad9535838ef459615
> Author: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
> Date:   Tue Dec 11 17:43:15 2007 -0500
> 
>     blk_end_request: changing DAC960 (take 4)
>     
>     This patch converts DAC960 to use blk_end_request interfaces.
>     Related 'UpToDate' arguments are converted to 'Error'.
>     
>     Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
>     Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
>     Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> 
> :100644 100644 9030c37... cd03473... M  drivers/block/DAC960.c
> 
> commit 117636092a87a28a013a4acb5de5492645ed620f
> Author: Ralf Baechle <ralf@linux-mips.org>
> Date:   Tue Oct 23 20:42:11 2007 +0200
> 
>     [PATCH] Fix breakage after SG cleanups
> 
> and I don't see how it could cause this.  The breakage is probably
> external to the driver.
> 
> I don't know what it could be and I don't know anyone who can be asked
> to look into it.
> 
> If you have time, the only way I can think of getting to the bottom of
> this is if you were to run a git bisection search as per
> http://www.kernel.org/doc/local/git-quick.html.  Sorry.  

Well, the DAC960 is very old.  It has a trick we escaped from in SCSI
where if it gets an error in the request it resubmits it a sector at a
time.  It sounds very much like it's doing that for every request if the
I/O speed is down to a few k/s.

So, could you try this patch?  It won't fix anything, but if the message
spews all over the console, we know the 1 sector at a time retry is
causing the problems.  If not we'll try to think of something else ...

James

---

diff --git a/drivers/block/DAC960.c b/drivers/block/DAC960.c
index cd03473..6e2c0e1 100644
--- a/drivers/block/DAC960.c
+++ b/drivers/block/DAC960.c
@@ -3410,6 +3410,10 @@ static void DAC960_queue_partial_rw(DAC960_Command_T *Command)
   struct request *Request = Command->Request;
   struct request_queue *req_q = Controller->RequestQueue[Command->LogicalDriveNumber];
 
+  if (printk_ratelimit())
+	  printk(KERN_ERR "DAC960 rety in single sector chunks, %llu:%lu\n",
+		 (u64)Request->sector, Request->nr_sectors);
+
   if (Command->DmaDirection == PCI_DMA_FROMDEVICE)
     Command->CommandType = DAC960_ReadRetryCommand;
   else
Comment 3 Anonymous Emailer 2008-05-28 11:37:37 UTC 
Reply-To: jens.axboe@oracle.com

On Wed, May 28 2008, James Bottomley wrote:
> On Wed, 2008-05-28 at 10:58 -0700, Andrew Morton wrote:
> > On Wed, 28 May 2008 03:52:37 -0700 (PDT) bugme-daemon@bugzilla.kernel.org
> wrote:
> > 
> > > http://bugzilla.kernel.org/show_bug.cgi?id=10810
> > > 
> > >            Summary: Performance regression on DAC960 and kernel 2.6.24+
> > >            Product: IO/Storage
> > >            Version: 2.5
> > >      KernelVersion: 2.6.24, 2.6.25
> > >           Platform: All
> > >         OS/Version: Linux
> > >               Tree: Mainline
> > >             Status: NEW
> > >           Severity: high
> > >           Priority: P1
> > >          Component: Block Layer
> > >         AssignedTo: axboe@kernel.dk
> > >         ReportedBy: alex@nibbles.it
> > > 
> > > 
> > > Latest working kernel version:
> > > 2.6.23
> > > 
> > > Earliest failing kernel version:
> > > 2.6.24
> > > 
> > > Distribution:
> > > Debian
> > > 
> > > Hardware Environment:
> > > 00:00.0 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
> > > 00:00.1 RAM memory: nVidia Corporation C51 Memory Controller 0 (rev a2)
> > > 00:00.2 RAM memory: nVidia Corporation C51 Memory Controller 1 (rev a2)
> > > 00:00.3 RAM memory: nVidia Corporation C51 Memory Controller 5 (rev a2)
> > > 00:00.4 RAM memory: nVidia Corporation C51 Memory Controller 4 (rev a2)
> > > 00:00.5 RAM memory: nVidia Corporation C51 Host Bridge (rev a2)
> > > 00:00.6 RAM memory: nVidia Corporation C51 Memory Controller 3 (rev a2)
> > > 00:00.7 RAM memory: nVidia Corporation C51 Memory Controller 2 (rev a2)
> > > 00:02.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> > > 00:03.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> > > 00:04.0 PCI bridge: nVidia Corporation C51 PCI Express Bridge (rev a1)
> > > 00:05.0 VGA compatible controller: nVidia Corporation C51G [GeForce 6100]
> (rev
> > > a2)
> > > 00:09.0 RAM memory: nVidia Corporation MCP51 Host Bridge (rev a2)
> > > 00:0a.0 ISA bridge: nVidia Corporation MCP51 LPC Bridge (rev a2)
> > > 00:0a.1 SMBus: nVidia Corporation MCP51 SMBus (rev a2)
> > > 00:0b.0 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
> > > 00:0b.1 USB Controller: nVidia Corporation MCP51 USB Controller (rev a2)
> > > 00:0d.0 IDE interface: nVidia Corporation MCP51 IDE (rev a1)
> > > 00:0e.0 IDE interface: nVidia Corporation MCP51 Serial ATA Controller
> (rev a1)
> > > 00:10.0 PCI bridge: nVidia Corporation MCP51 PCI Bridge (rev a2)
> > > 00:14.0 Bridge: nVidia Corporation MCP51 Ethernet Controller (rev a1)
> > > 00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> > > HyperTransport Technology Configuration
> > > 00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> Address
> > > Map
> > > 00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> DRAM
> > > Controller
> > > 00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron]
> > > Miscellaneous Control
> > > 04:08.0 RAID bus controller: Mylex Corporation AcceleRAID 352/170/160
> support
> > > Device (rev 02)
> > > 
> > > Software Environment:
> > > Debian Lenny 64bit
> > > 
> > > Problem Description:
> > > I/O Access is very slow on some condition, for example samba users can't
> write
> > > more than a few KB/sec on the shares.
> > > Also tomcat is veeeery slow to startup (at least 3 times the normal
> time).
> > > 
> > > Steps to reproduce:
> > > Simply boot with the new kernel
> > 
> > Oh dear.
> > 
> > There's been only one change to DAC960.c in that timeframe:
> > 
> > commit 0156c2547e92df559d5592aad9535838ef459615
> > Author: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
> > Date:   Tue Dec 11 17:43:15 2007 -0500
> > 
> >     blk_end_request: changing DAC960 (take 4)
> >     
> >     This patch converts DAC960 to use blk_end_request interfaces.
> >     Related 'UpToDate' arguments are converted to 'Error'.
> >     
> >     Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
> >     Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
> >     Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> > 
> > :100644 100644 9030c37... cd03473... M  drivers/block/DAC960.c
> > 
> > commit 117636092a87a28a013a4acb5de5492645ed620f
> > Author: Ralf Baechle <ralf@linux-mips.org>
> > Date:   Tue Oct 23 20:42:11 2007 +0200
> > 
> >     [PATCH] Fix breakage after SG cleanups
> > 
> > and I don't see how it could cause this.  The breakage is probably
> > external to the driver.
> > 
> > I don't know what it could be and I don't know anyone who can be asked
> > to look into it.
> > 
> > If you have time, the only way I can think of getting to the bottom of
> > this is if you were to run a git bisection search as per
> > http://www.kernel.org/doc/local/git-quick.html.  Sorry.  
> 
> Well, the DAC960 is very old.  It has a trick we escaped from in SCSI
> where if it gets an error in the request it resubmits it a sector at a
> time.  It sounds very much like it's doing that for every request if the
> I/O speed is down to a few k/s.
> 
> So, could you try this patch?  It won't fix anything, but if the message
> spews all over the console, we know the 1 sector at a time retry is
> causing the problems.  If not we'll try to think of something else ...

A bit unlikely, me thinks...

Anyway, a blktrace dump of some IO would show what is going on. I'm
assuming the problem is persistent across IO schedulers?
Comment 4 Anonymous Emailer 2008-05-28 12:57:29 UTC 
Reply-To: James.Bottomley@HansenPartnership.com

On Wed, 2008-05-28 at 20:37 +0200, Jens Axboe wrote:
> On Wed, May 28 2008, James Bottomley wrote:
> > Well, the DAC960 is very old.  It has a trick we escaped from in SCSI
> > where if it gets an error in the request it resubmits it a sector at a
> > time.  It sounds very much like it's doing that for every request if the
> > I/O speed is down to a few k/s.
> > 
> > So, could you try this patch?  It won't fix anything, but if the message
> > spews all over the console, we know the 1 sector at a time retry is
> > causing the problems.  If not we'll try to think of something else ...
> 
> A bit unlikely, me thinks...

I can't really see any other way of getting such a massive slowdown ...
but give us your straws, we can grasp at them too ...

> Anyway, a blktrace dump of some IO would show what is going on. I'm
> assuming the problem is persistent across IO schedulers?

Yes, that might help.  If it's not the one sector chunk problem it would
have to be either some strange wait issue or massive retries.

James
Comment 5 Anonymous Emailer 2008-05-29 01:03:48 UTC 
Reply-To: jens.axboe@oracle.com

On Wed, May 28 2008, bugme-daemon@bugzilla.kernel.org wrote:
> http://bugzilla.kernel.org/show_bug.cgi?id=10810
> 
> 
> 
> 
> 
> ------- Comment #4 from anonymous@kernel-bugs.osdl.org  2008-05-28 12:57
> -------
> Reply-To: James.Bottomley@HansenPartnership.com
> 
> On Wed, 2008-05-28 at 20:37 +0200, Jens Axboe wrote:
> > On Wed, May 28 2008, James Bottomley wrote:
> > > Well, the DAC960 is very old.  It has a trick we escaped from in SCSI
> > > where if it gets an error in the request it resubmits it a sector at a
> > > time.  It sounds very much like it's doing that for every request if the
> > > I/O speed is down to a few k/s.
> > > 
> > > So, could you try this patch?  It won't fix anything, but if the message
> > > spews all over the console, we know the 1 sector at a time retry is
> > > causing the problems.  If not we'll try to think of something else ...
> > 
> > A bit unlikely, me thinks...
> 
> I can't really see any other way of getting such a massive slowdown ...
> but give us your straws, we can grasp at them too ...

You are right, something must be going fundementally wrong for
such a slow down to happen. The reason I think it's unlikely is
because the retries would be happening in earlier kernels as
well, so it should not show up as a regression.

> > Anyway, a blktrace dump of some IO would show what is going on. I'm
> > assuming the problem is persistent across IO schedulers?
> 
> Yes, that might help.  If it's not the one sector chunk problem it
> would have to be either some strange wait issue or massive retries.

Yep. To the reporter - is the slowdown associated with excessive
CPU usage (system or otherwise), or is it just slow IO?
Comment 6 Alessandro Polverini 2008-09-20 01:48:35 UTC 
Problem seems gone with 2.6.26, at least it does not exhibit with debian kernel 
linux-image-2.6.26-1-amd64 version 2.6.26-4