Re: [Xen-devel] VT is comically slow

[Rik van Riel <riel@xxxxxxxxxx>]

Daniel Veillard wrote:
> On Mon, Jul 03, 2006 at 09:58:01AM +0100, Keir Fraser wrote:
> > On 3 Jul 2006, at 09:48, Rik van Riel wrote:
> >
> > > Looking at it a bit more closely, it appears that postgresql
> > > doing disk IO from inside a fully virtualized domain totally
> > > kills the CPU.
> > >
> > > It gets so bad that a simple "dmesg" takes 10-20 seconds to start,
> > > and after that it spews data maybe 7 or 8 lines every other second.
> > > Actually slower than serial console...
> > >
> > > This is totally unusable :(
> > Might you be emulating PIO? That would certainly suck. The device model 
> > is supposed to support (virtual) DMA though.
>   That's the first thing I though about but apparently the kernel
> runs the IDE device in DMA mode if one believe the hdparm output in
> that guest.
The information is conflicting...

While hdparm suggests DMA, the kernel profile has a suspicious
amount of IDE in it.  Not sure what's going on...

# hdparm /dev/hda

/dev/hda:
 multcount    = 16 (on)
 IO_support   =  0 (default 16-bit)
 unmaskirq    =  0 (off)
 using_dma    =  1 (on)
 keepsettings =  0 (off)
 readonly     =  0 (off)
 readahead    = 256 (on)
 geometry     = 65535/16/63, sectors = 85899345920, start = 0


# readprofile | sort -n | tail -20
148632 copy_user_generic                        498.7651
163787 copy_page                                731.1920
178136 __might_sleep                            1017.9200
178409 ide_intr                                 141.7069
277185 ide_outsl                                39597.8571
324592 ide_execute_command                      772.8381
349538 sys_select                               300.0326
440036 unmap_vmas                               264.6037
478669 do_page_fault                            303.7240
585427 handle_IRQ_event                         6887.3765
1087766 ide_inw                                  120862.8889
1115456 ide_do_request                           950.1329
2169643 do_wp_page                               1662.5617
2945913 ide_outbsync                             736478.2500
3888598 i8042_interrupt                          6480.9967
5892214 do_no_page                               2986.4237
11803041 thread_return                            20890.3381
13074317 __do_softirq                             80705.6605
81661803 poll_idle                                983877.1446
130083897 total                                     52.4560



--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan

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