UL: logging before initialization?

[David Holmes]

Hi Thomas,

Expanding to hotspot-dev as UL was not designed/built by the runtime team.

My comments below based on my limited understanding of UL.

On 11/03/2016 2:25 AM, Thomas Stüfe wrote:
> Hi all,
>
> I wanted to use UL to log some information from early OS initialization but
> found that we initialize logging quite late. It is initialized way after
> os::init() runs.

There are various dependencies in the unified logging framework that 
prevent it (or at least parts of it) from being used early eg:
- argument parsing and log configuration
- existence of current thread (for logStreams)

> I also see that warnings and errors are printed unconditionally even
> before, only log calls for levels <= info are quietly swallowed.

Are those warnings/errors using UL? I suspect not.

Silent swallowing is not good though.

> Would there be a way to move UL initialization and parsing of -Xlog
> Arguments to an earlier point to be able to use logging in os::init()?

I don't see how you could utilize tags and levels until after full 
initialization of the logging system, so that would preclude using it as 
early as os::init.

> Or, if that is not possible, would there be a way to delay printing of
> early logging calls to after logging initialization and then print them out
> in one go? I think this would be not trivial though: one would have to
> collect all logging output in memory somewhere - because we do not yet know
> which tags are switched on - and then, once -Xlog is parsed, filter it by
> tag before printing it out.

Yeah that seems difficult/complex given the problem that the VM is in 
the early stages of initialization

> What do you think, would that be a worthwhile change? I am a bit unhappy
> about how logging calls are quietly swallowed, so for any code which runs
> at VM initialization, one always has to keep in mind that UL may not yet be
> initialized.

Not a new problem (adding more code during VM initialization) but one 
that is getting more complicated :)

Cheers,
David

> Kind Regards, Thomas
>