Re: kvm-27 vs 28 I/O speed

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Dave Hansen wrote:
> On Fri, 2007-07-13 at 16:41 +0300, Avi Kivity wrote:
>   
>> I tested FC6 i386 about found no difference with or without the patch.
>>     
>
> I've git bisected down the the same patch that you pasted above.  I've
> also tried just applying that single patch to kvm-27 and reproduced the
> slow behavior.
>
>   
>> Dave, can you diff the guest dmesg with kvm-27 and kvm-28 userspace? 
>> Maybe that will tell us something.
>>     
>
> Here's kvm-27 and kvm-27+55a3212bc2...:
>
>
> diff -ru kvm-fast-dmesg.txt kvm-slow-dmesg.txt 
>
>  Linux version 2.6.22 (dave@kernel) (gcc version 4.1.2 (Ubuntu 4.1.2-0ubuntu4)) #13 Wed Jul 11 15:27:01 PDT 2007
>  BIOS-provided physical RAM map:
> @@ -39,7 +22,7 @@
>  Enabling unmasked SIMD FPU exception support... done.
>  Initializing CPU#0
>  PID hash table entries: 512 (order: 9, 2048 bytes)
> -Detected 2669.668 MHz processor.
> +Detected 2669.011 MHz processor.
>  Console: colour VGA+ 80x25
>  Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
>  Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
> @@ -53,7 +36,7 @@
>        .data : 0xc036efcf - 0xc043911c   ( 808 kB)
>        .text : 0xc0100000 - 0xc036efcf   (2491 kB)
>  Checking if this processor honours the WP bit even in supervisor mode... Ok.
> -Calibrating delay using timer specific routine.. 26758.82 BogoMIPS (lpj=53517640)
> +Calibrating delay using timer specific routine.. 26800.00 BogoMIPS (lpj=53600004)
>  Mount-cache hash table entries: 512
>  CPU: L1 I cache: 8K
>  CPU: L2 cache: 128K
> @@ -107,20 +90,21 @@
>  PIIX3: IDE controller at PCI slot 0000:00:01.1
>  PIIX3: chipset revision 0
>  PIIX3: not 100% native mode: will probe irqs later
> -PIIX3: neither IDE port enabled (BIOS)
> +    ide0: BM-DMA at 0x1400-0x1407, BIOS settings: hda:pio, hdb:pio
> +    ide1: BM-DMA at 0x1408-0x140f, BIOS settings: hdc:pio, hdd:pio
>   

There's your problem.  With that patch, your kernel is using PIO instead 
of DMA.

I'm not 100% sure what's going here other than the fact that this 
particular problem has cropped up a number of times on qemu-devel.  It's 
been very difficult to get all guests to actually use DMA.

Regards,

Anthony Liguori

>  hda: QEMU HARDDISK, ATA DISK drive
>  hdb: QEMU HARDDISK, ATA DISK drive
> -Clocksource tsc unstable (delta = 767711774 ns)
> +ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
> +Clocksource tsc unstable (delta = 768021683 ns)
>  Time: pit clocksource has been installed.
>  hdc: QEMU CD-ROM, ATAPI CD/DVD-ROM drive
> -ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
>  ide1 at 0x170-0x177,0x376 on irq 15
>  hda: max request size: 512KiB
> -hda: 10485760 sectors (5368 MB) w/256KiB Cache, CHS=10402/255/63
> +hda: 10485760 sectors (5368 MB) w/256KiB Cache, CHS=10402/255/63, (U)DMA
>  hda: cache flushes supported
>   hda: hda1
>  hdb: max request size: 512KiB
> -hdb: 262144 sectors (134 MB) w/256KiB Cache, CHS=260/255/63
> +hdb: 262144 sectors (134 MB) w/256KiB Cache, CHS=260/255/63, (U)DMA
>  hdb: cache flushes supported
>   hdb: hdb1
>  serio: i8042 KBD port at 0x60,0x64 irq 1
> @@ -146,7 +130,7 @@
>  Adding 128480k swap on /dev/hdb1.  Priority:-1 extents:1 across:128480k
>  Checking root file system...
>  fsck 1.37 (21-Mar-2005)
> -/dev/hda1: clean, 46970/655360 files, 287821/1309289 blocks
> +/dev/hda1: clean, 46970/655360 files, 287829/1309289 blocks
>  EXT3 FS on hda1, internal journal
>  Cleaning up ifupdown...done.
>  Checking all file systems...
>
> Looks like the slow versions have DMA enabled.  Is it slower to emulate
> DMA than PIO?
>
> -- Dave
>
>
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>   


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