[ovs-dev] [PATCH v3] timeval: Use monotonic time in OVS Python timeval library.

[YAMAMOTO Takashi]

hi,

> Hey Yamamoto,
> 
> Sorry for the delay, I finally set up my NetBSD environment and tested
> this script. You're right, but NetBSD will actually return an error on
> clock_gettime because of this argument difference. Thus, NetBSD will
> continue to use time.time(), which is as expected.

it returns an error because of a different value of CLOCK_MONOTONIC.
(it's 3 on NetBSD)

however, it's too fragile to pass "random" arguments to a system call
and expect it detect and return errors.
how about having an explicit platform check?

> 
> I can add NetBSD functionality such that clock_gettime with
> CLOCK_MONOTONIC can also be used for NetBSD as well. Lemme know!

thanks.  please if you can.  otherwise i can take a look later.

btw, docstring of msec() needs an update.

YAMAMOTO Takashi

> 
> Ryan
> 
> On 5/31/14 11:14 PM, "YAMAMOTO Takashi" <yamamoto at valinux.co.jp> wrote:
> 
>>> Python's time.time() function uses the system wall clock. However,
>>> if NTP resets the wall clock to be a time in the past, then this
>>> causes any applications that poll block based on time, such as
>>> ovs-xapi-sync, to poll block indefinitely since the time is
>>> unexpectedly negative.
>>> 
>>> This patch fixes the issue by using time.monotonic() if Python's
>>> version >= 3.3. Otherwise, the timeval module calls out to the
>>> librt C shared library and uses the clock_gettime function with
>>> CLOCK_MONOTONIC.
>>> 
>>> Note this is only enabled on Linux-based platforms. This has been
>>> tested on Ubuntu 12.04 and Redhat 6.4.
>>> 
>>> Bug #1189434
>>> Signed-off-by: Ryan Wilson <wryan at nicira.com>
>>> 
>>> ---
>>> v2: Pre-load librt.so.1 library instead of loading at every function
>>> call. Use only CLOCK_MONOTONIC for consistency with OVS C library.
>>> v3: Edit commit message, remove if-linux check
>>> ---
>>>  python/ovs/timeval.py |   34 +++++++++++++++++++++++++++++++++-
>>>  1 file changed, 33 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/python/ovs/timeval.py b/python/ovs/timeval.py
>>> index ba0e54e..f2681ac 100644
>>> --- a/python/ovs/timeval.py
>>> +++ b/python/ovs/timeval.py
>>> @@ -12,13 +12,45 @@
>>>  # See the License for the specific language governing permissions and
>>>  # limitations under the License.
>>>  
>>> +import ctypes
>>> +import sys
>>>  import time
>>>  
>>> +LIBRT = 'librt.so.1'
>>> +CLOCK_MONOTONIC = 1
>>> +
>>> +class timespec(ctypes.Structure):
>>> +    _fields_ = [
>>> +        ('tv_sec', ctypes.c_long),
>>
>>on NetBSD, timespec.tv_sec is int64_t, not long.
>>
>>YAMAMOTO Takashi
>>
>>> +        ('tv_nsec', ctypes.c_long),
>>> +    ]
>>> +
>>> +try:
>>> +    librt = ctypes.CDLL(LIBRT)
>>> +    clock_gettime = librt.clock_gettime
>>> +    clock_gettime.argtypes = [ctypes.c_int, ctypes.POINTER(timespec)]
>>> +except:
>>> +    # Librt shared library could not be loaded
>>> +    librt = None
>>> +
>>> +def monotonic():
>>> +    if not librt:
>>> +        return time.time()
>>> +
>>> +    t = timespec()
>>> +    if clock_gettime(CLOCK_MONOTONIC, ctypes.pointer(t)) == 0:
>>> +        return t.tv_sec + t.tv_nsec * 1e-9
>>> +    # Kernel does not support CLOCK_MONOTONIC
>>> +    return time.time()
>>> +
>>> +# Use time.monotonic() if Python version >= 3.3
>>> +if not hasattr(time, 'monotonic'):
>>> +    time.monotonic = monotonic
>>>  
>>>  def msec():
>>>      """Returns the current time, as the amount of time since the
>>>epoch, in
>>>      milliseconds, as a float."""
>>> -    return time.time() * 1000.0
>>> +    return time.monotonic() * 1000.0
>>>  
>>>  
>>>  def postfork():
>>> -- 
>>> 1.7.9.5
> 
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