Use monotonic/steady clock by default if available

[hololqq]

Hi,

I'm new to use the haivision srt project and have a question.

Out product uses v1.4.1 to achieve SRT transmit, but we found a bug that SRT will drop packet when system time is changed via NTP/manual. I checked the code and I though the related code is gettimeofday(), which use the system time as SRT start time.

Today I found the release logs of the 1.4.2&1.4.3, and some logs related to this function. So I imagined there's an issue indicated this bug and had been resolved in the following releases. Is this right? If so, please give me a reply.

Thanks for your time:)

[maxsharabayko]

Hi @hololqq
SRT v1.4.2 adds two options for SRT to use monotonic time source:

1. C++11 std::chrono::steady_clock can be enabled with -DENABLE_STDCXX_SYNC=ON CMake build option.
2. POSIX clock_gettime with monotonic time source can be enabled with -DENABLE_MONOTONIC_CLOCK.

If you can use C++11 compliant compiler, consider using the first option.

[hololqq]

Hi @hololqq
SRT v1.4.2 adds two options for SRT to use monotonic time source:

1. C++11 std::chrono::steady_clock can be enabled with -DENABLE_STDCXX_SYNC=ON CMake build option.
2. POSIX clock_gettime with monotonic time source can be enabled with -DENABLE_MONOTONIC_CLOCK.

If you can use C++11 compliant compiler, consider using the first option.

Thanks. I'll test it today.

[gizahNL]

Hi @hololqq
SRT v1.4.2 adds two options for SRT to use monotonic time source:

1. C++11 std::chrono::steady_clock can be enabled with -DENABLE_STDCXX_SYNC=ON CMake build option.
2. POSIX clock_gettime with monotonic time source can be enabled with -DENABLE_MONOTONIC_CLOCK.

If you can use C++11 compliant compiler, consider using the first option.

What is the rationale for not having these turned on by default? It would seem to me that monotonic clock source is a strict requirement for a networking protocol doing time based recovery.

[maxsharabayko]

@gizahNL

What is the rationale for not having these turned on by default?

The end goal is to fully transition to a monotonic clock. There are/were several things to consider before transitioning.

1. Compatibility with old versions of SRT using wallclock. This has been confirmed. ✔️
2. Define what should be the new defaults.

As shared above, there are two ways to enable a monotonic clock:

1. C++11 std::chrono::steady_clock can be enabled with -DENABLE_STDCXX_SYNC=ON CMake build option.
2. POSIX clock_gettime with monotonic time source can be enabled with -DENABLE_MONOTONIC_CLOCK.

Without those options, the current defaults are:

• On MacOS SRT uses mach_absolute_time(..) by default, which is monotonic, except the timer is not incremented while the device is asleep.
• On Windows QueryPerformanceCounter(..) is used by default, though together with gettimeofday() port in pthreads4win. Not 100% accurate monotonic clock.
• On Linux gettimeofday(..) is used by default. Wall clock.

The New Defaults to Apply

There are two options here.

1. The easiest way is to enable C++11 build of SRT by default, providing STL steady clock. However, it may impact some integrations, forcing them to explicitly disable C++11 in SRT. Also, in some experiments, we observed STL steady clock to provide system clock, which is weird and requires further study.

2. Another option is to enable C++11 by default only for Windows builds, and enable -DENABLE_MONOTONIC_CLOCK for Linux builds. If a Linux system does not support CLOCK_MONOTONIC, it is the user who should specify -DENABLE_MONOTONIC_CLOCK=OFF.