Subject: Problem with clock_gettime()
To: netbsd user <netbsd-users@netbsd.org>
From: Ian McIntosh <ianm@cat.co.za>
List: netbsd-users
Date: 11/24/2003 10:18:22
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Hi
I have encountered an interesting problem in NetBSD 1.5.1 with =
clock_gettime.=20
I am currently running tests with a custom designed PCI frame grabber. =
My current
test is to stress the PCI bus a little to try and determine the maximum =
DMA transfer
rate I could possibly get. When doing this I have noticed some strange =
behaviour
with clock_gettime(). I can call clock_gettime() and store the result in =
a temporary=20
variable and then when I call it again the time returned is before the =
time in the=20
temporary variable (indicating that time has gone backwards????). The =
time difference
is only ever a few milli-seconds. I have even tested this when the bus =
is not that stressed
but is still fairly busy. The problem only appears when the bus is busy =
for extended periods=20
of time. When the bus is not busy then everything seems fine.
Here is the core of the test program I run.
timespec now,old;
clock_gettime(CLOCK_REALTIME, &now)
old=3Dnow;
while (1)
{
clock_gettime(CLOCK_REALTIME, &now);
if (now<old)
assert; // asserts and stops program.......time has gone =
backwards
old=3Dnow;
}
I have operators to correctly compare timespec values and make them =3D =
and so on.
Anyone have any ideas that can explain this? If so I would very much =
like to know.
Thanks=20
Ian
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<DIV><FONT face=3DArial size=3D2>Hi</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>I have encountered an interesting =
problem in NetBSD=20
1.5.1 with clock_gettime. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2>I am currently running tests with a =
custom designed=20
PCI frame grabber. My current</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>test is to stress the PCI bus a little =
to try and=20
determine the maximum DMA transfer</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>rate I could possibly get. When doing =
this I have=20
noticed some strange behaviour</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>with clock_gettime(). I can call =
clock_gettime()=20
and store the result in a temporary </FONT></DIV>
<DIV><FONT face=3DArial size=3D2>variable and then when I call it again =
the time=20
returned is before the time in the </FONT></DIV>
<DIV><FONT face=3DArial size=3D2>temporary variable (indicating that =
time has gone=20
backwards????). The time difference</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>is only ever a few milli-seconds. I =
have even=20
tested this when the bus is not that stressed</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>but is still fairly busy. The problem =
only appears=20
when the bus is busy for extended periods </FONT></DIV>
<DIV><FONT face=3DArial size=3D2>of time. </FONT><FONT face=3DArial =
size=3D2>When the=20
bus is not busy then everything seems fine.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Here is the core of the test program I=20
run.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>timespec now,old;</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>clock_gettime(CLOCK_REALTIME,=20
&now)</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>old=3Dnow;</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>while (1)</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>{</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> =
clock_gettime(CLOCK_REALTIME,=20
&now);</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> if =
(now<old)</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> =20
assert; // asserts and stops program.......time has gone=20
backwards</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> =
old=3Dnow;</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>}</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>I have operators to correctly compare =
timespec=20
values and make them =3D and so on.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Anyone have any ideas that can explain =
this? If so=20
I would very much like to know.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Thanks </FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Ian</FONT></DIV></BODY></HTML>
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