RFR: 8185925 & 8153682 (footprint reduction of java.lang.StackFrameInfo)

[Remi Forax]

On November 7, 2017 8:48:43 PM GMT+01:00, Vitaly Davidovich <vitalyd at gmail.com> wrote:
>On Tue, Nov 7, 2017 at 1:25 PM Remi Forax <forax at univ-mlv.fr> wrote:
>
>> My bad,
>> I've calculated that the header was 8 bytes instead of 12, so I've
>> supposed that the boolean was not stored in a byte.
>>
>> For my culture, why the header is 12 bytes, the pointer to the vtable
>is
>> on 64bits and can not be compressed like the other oops ?
>
>It’s 12 bytes (on x64) with compressed klass ptr; 8 bytes for the mark
>word
>and a 4 byte compressed klass ptr.  It’s 16 bytes without compressed
>klass
>ptr.  Or are you asking something else Remi?

My question is more given you have compressed oops  (and compressed klass) why the header is 12 bytes and not 8 bytes (a two words header).

so why the  mark word is 8 bytes and not 4 ? At worst it's a pointer and again you can use the compressed oops trick ?

>
>I think many folks would be upset if a boolean wasn’t stored as a byte
>:).

yes, right.

Remi

>
>>
>>
>> Rémi
>>
>>
>> On November 7, 2017 5:42:33 PM GMT+01:00, Brent Christian <
>> brent.christian at oracle.com> wrote:
>> >Hi, Remi
>> >
>> >Thanks for the idea.  From my reading of the jol ouput:
>> >
>> >
>>
>http://cr.openjdk.java.net/~bchristi/8185925/StackFrameInfo.jol.rmDeclClass.txt
>> >
>> >
>> >
>> >I believe retainClassRef is already being stored in a single byte
>(i.e.
>> >
>> >SIZE of 1):
>> >
>> >  OFFSET  SIZE     TYPE DESCRIPTION
>> >...
>> >      10     1  boolean StackFrameInfo.retainClassRef
>> >
>> >Thanks,
>> >-Brent
>> >
>> >On 11/6/17 11:45 PM, Remi Forax wrote:
>> >> Hi Brent,
>> >> if you declare retainClassRef as a byte, you can even compress the
>> >data structure a little bit more, no ?
>> >> a boolean (as a field) uses 4 bytes while a byte uses welll 1
>byte.
>> >> Given that bci is a short, the VM will put both bci and
>> >retainClassRef on the same 32 bits slot.
>> >>
>> >> cheers,
>> >> Rémi
>> >>
>> >> ----- Mail original -----
>> >>> De: "Brent Christian" <brent.christian at oracle.com>
>> >>> À: "core-libs-dev" <core-libs-dev at openjdk.java.net>,
>"hotspot-dev"
>> ><hotspot-dev at openjdk.java.net>
>> >>> Envoyé: Mardi 7 Novembre 2017 01:23:16
>> >>> Objet: RFR: 8185925 & 8153682 (footprint reduction of
>> >java.lang.StackFrameInfo)
>> >>
>> >>> Hi,
>> >>>
>> >>> There are a couple opportunities to reduce the memory footprint
>of
>> >>> java.lang.StackFrameInfo (the internal implementation of
>> >>> java.lang.StackWalker.StackFrame):
>> >>>
>> >>> 8153682[1] : StackFrameInfo.declaringClass could be removed
>> >>> 8185925[2] : StackFrameInfo::walker field can be replaced with
>> >bitmap to
>> >>> save footprint
>> >>>
>> >>> I had a look using jol[3].  Removing only 'walker' helps only
>under
>> >32-
>> >>> and 64-bit, but not with compressed oops.  Removing both 'walker'
>> >and
>> >>> 'declaringClass' brings a benefit to compressed oops as well
>(though
>> >not
>> >>> for 16-byte aligned).
>> >>>
>> >>> The size change, in bytes, for each execution mode is as follows:
>> >>>
>> >>>                               32-bit: 32->24
>> >>>                               64-bit: 56->40
>> >>>                   64/compressed oops: 32->24
>> >>> 64/compressed oops, 16-byte aligned: 32->32
>> >>>
>> >>> (For reference, the jol reports for the baseline and specimen are
>at
>> >[4]
>> >>> and [5], respectively.)
>> >>>
>> >>> Please review my code change for this.  The webrev is here:
>> >>> http://cr.openjdk.java.net/~bchristi/8185925/webrev.03/
>> >>>
>> >>> An automated test run is in progress.
>> >>>
>> >>> Thanks!
>> >>> -Brent
>> >>>
>> >>> --
>> >>> 1. https://bugs.openjdk.java.net/browse/JDK-8153682
>> >>> 2. https://bugs.openjdk.java.net/browse/JDK-8185925
>> >>> 3. http://openjdk.java.net/projects/code-tools/jol/
>> >>> 4.
>> >>>
>> >
>>
>http://cr.openjdk.java.net/~bchristi/8185925/StackFrameInfo.jol.baseline.txt
>> >>> 5.
>> >>>
>> >
>>
>http://cr.openjdk.java.net/~bchristi/8185925/StackFrameInfo.jol.rmDeclClass.txt
>>
>> --
>> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>>

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.