JEP 271: Unified GC Logging - Second pre-review

Vitaly Davidovich vitalyd at gmail.com
Tue Nov 10 17:19:47 UTC 2015


I think it's more of whether the GC log info is noise or signal.  If a
process is GC'ing very frequently, looking at it manually doesn't strike me
as practical -- you'll need a tool to make sense of it (i.e. each GC
occurrence is noise).

On Tue, Nov 10, 2015 at 12:16 PM, charlie hunt <charlie.hunt at oracle.com>
wrote:

>
> On Nov 10, 2015, at 11:02 AM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
>
> 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.
>
>
> I may not be the norm ... I almost always look at GC logs manually.
>
> And in those cases where look at GC logs via tools / visualization I
> always end up looking at the logs to see was happening leading up to some
> GC event of interest.
>
> That said ... on the one hand I agree that many folks don't read GC logs,
> but on the other hand, for my selfish purposes, I look at them, and I'd
> like something that's easy to read. (Put whatever definition you'd like on
> "easy to read").
>
> Don't know if that helps or complicates things for Bengt?
>
> Charlie
>
> 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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