JEP 271: Unified GC Logging - Second pre-review

Vitaly Davidovich vitalyd at gmail.com
Tue Nov 10 17:02:52 UTC 2015


>
> I’m sorry to say but what you are saying just doesn’t correlate well with
> my experience. In my experience people rarely look at GC logging. That
> said, you are right, readability is not overly important to me however
> being completely unreadable wouldn’t be great either because I also open
> them up and read them.


I do occasionally look at GC logs manually.  The reason is because GC is
unexpected and/or rare event on some daemons.  I agree that on daemons
where GC is a frequent occurrence nobody is likely to look at it manually
unless they're examining a particular outlier.

On Tue, Nov 10, 2015 at 10:51 AM, Kirk Pepperdine <kirk.pepperdine at gmail.com
> wrote:

>
> >>>>
> >>>>>> [11.247s][info   ][gc,heap     ] GC#265 38912K->0K(2048K)
> 4096K->4096K(6144K) 68990K->73147K(131072K) 2993K->2993K(1056768K)
> >>> This format may be fine for machine parsing but it is not very
> readable to humans. Readability is an important use case,
> >> Trust me on this one... no one reads these things. Even when I tell
> attendees of my workshop to read the GC log, they still don’t. Readability
> might be an important an interesting use case from your POV. From mine it
> really isn’t. As an FYI, you should look at HP and Azul GC logs.
> >
> > I understand that readability is not important to you. But there are
> many other users than you and many of them depend on being able to read the
> GC logs in a reasonable way.
>
> I’m sorry to say but what you are saying just doesn’t correlate well with
> my experience. In my experience people rarely look at GC logging. That
> said, you are right, readability is not overly important to me however
> being completely unreadable wouldn’t be great either because I also open
> them up and read them.
>
> >
> >>
> >>> so we will need to at least add information about what the numbers map
> to. As I stated in a previous mail I would prefer these on separate lines,
> but if we want them on a single line I think the format need to be
> something like:
> >>>
> >>> [11.247s][info   ][gc,heap     ] GC#265 Eden: 38912K->0K(2048K),
> Survivor: 4096K->4096K(6144K), Old: 68990K->73147K(131072K), Metaspace:
> 2993K->2993K(1056768K)
> >>>
> >>> To me this is line is too long to ready quickly. In particular if I am
> just trying to follow how, for example, the Eden size changes over time.
> >> My concern is here and why I believe they should be combined is to
> minimize the frequency of logging.
> >
> > The frequency as in logging three lines instead of one line per GC? You
> would have to show me some performance numbers if you want to convince me
> that this would be a problem.
>
> This is a problem that you can’t see in the small. It is my experience
> tuning systems (sorry for the blatant hand-waving) that the way to choke a
> logging frame work is to have it work very frequently. The size of the
> message is secondary in that many small writes are far worse than fewer
> larger ones. For example, if you have a system that is logging to NAS on a
> system that is shy on network capacity, GC logging will degrade
> performance. Frequency in this situation hurts quite a bit. So, per GC
> logging, I’d agree with you 99% of the time. However it’s that 1% of the
> time that is important because those are the cases where you need the data
> and it very hard to get it when collecting it puts more pressure on the
> system.
>
> I find it very ironic that logging, which is intended to improve the
> visibility of your application, is so often a bottleneck in applications
> simply because the way it works isn’t visible.
>
> All said, I think I’ve offered all I can in terms of my experiences on
> this subject so no worries, I’ll just let you get on with it.
>
> Kind regards,
> Kirk
>
>
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