[ntp:questions] Re: IEEE 1588 support in NTP?

Bjorn Gabrielsson bg at lysator.liu.se
Thu Nov 3 08:03:45 UTC 2005

hmurray at suespammers.org (Hal Murray) writes:

> >I'd like to object: Some long time ago someone (I can't remember) made a test
> >over an extended period of time showing that Linux did not loose a single
> >interrupt. However depending on the drivers and crappy hardware (not to talk
> >about CPU frequency scaling and ACPI's various power saving modes), the system
> >may loose interrupts. For any hardware there is a rate of interrupts that
> >cannot be handled. For my 16MHz 386SX that rate was quite low.
> It may depend upon which version/distributino of Linux you are talking
> about.

> I have an old RH 7.2 system.  After a reasonably vanilla install,
> it was using PIO for disk accesses.  Any big burst of disk activity
> would cause missed interrupts.  I have a test hack for measuring
> disk bandwidth.  It would totally trash the clock.

> After poking around a bit I found some simple way to activate the
> DMA mechanism.  That solved the problem for me.

> I forget the details. 

'man hdparm'

Looking at my system, substitute '/dev/hdc' with where your disks are.

# hdparm /dev/hdc  
 multcount    = 16 (on)
 IO_support   =  1 (32-bit)
 unmaskirq    =  1 (on)
 using_dma    =  1 (on)                  <******* Shows DMA is on
 keepsettings =  0 (off)
 readonly     =  0 (off)
 readahead    = 256 (on)
 geometry     = 65535/16/63, sectors = 241254720, start = 0

# hdparm -i /dev/hdc   % Check where the '*' is in the list of IDE modes.
 Model=IC35L120AVVA07-0, FwRev=VA6OA52A, SerialNo=VNC607A6GLWZYJ
 Config={ HardSect NotMFM HdSw>15uSec Fixed DTR>10Mbs }
 RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=52
 BuffType=DualPortCache, BuffSize=1863kB, MaxMultSect=16, MultSect=16
 CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=241254720
 IORDY=on/off, tPIO={min:240,w/IORDY:120}, tDMA={min:120,rec:120}
 PIO modes:  pio0 pio1 pio2 pio3 pio4
 DMA modes:  mdma0 mdma1 mdma2
 UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5 
 DMA mode is used
 AdvancedPM=yes: disabled (255) WriteCache=enabled
 Drive conforms to: ATA/ATAPI-5 T13 1321D revision 1:
 * signifies the current active mode

To get DMA working you ofcause need to have support for your IDE
chipset in your kernel.


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