• bearophile (7/7) Mar 11 2012 A significant percentage of heap memory in a 64 bit JavaVM is used by re...
• =?ISO-8859-1?Q?Alex_R=F8nne_Petersen?= (12/19) Mar 11 2012 This probably can't work for D.
• deadalnix (4/33) Mar 11 2012 Additionally, 64bits pointer is good for us :

bearophile <bearophileHUGS lycos.com> writes:

A significant percentage of heap memory in a 64 bit JavaVM is used by
references, that are 8 bytes long. To reduce this problem they have invented
"compressed references" (useful only for 64 bit systems), that turn references
to 32 bit again:

https://blogs.oracle.com/jrockit/entry/understanding_compressed_refer

ftp://public.dhe.ibm.com/software/webserver/appserv/was/WAS_V7_64-bit_performance.pdf

With them the total amount of heap memory decreases. They don't slow down the
class usage significantly, and the memory reduction reduces the CPU cache
pressure, increasing the performance a bit.

Will this idea be useful for 64 bit D/DMD too, for D GC-managed class
references (not for raw pointers)?

Bye,
bearophile

=?ISO-8859-1?Q?Alex_R=F8nne_Petersen?= <xtzgzorex gmail.com> writes:

On 12-03-2012 01:41, bearophile wrote:
 A significant percentage of heap memory in a 64 bit JavaVM is used by
references, that are 8 bytes long. To reduce this problem they have invented
"compressed references" (useful only for 64 bit systems), that turn references
to 32 bit again:

 https://blogs.oracle.com/jrockit/entry/understanding_compressed_refer

 ftp://public.dhe.ibm.com/software/webserver/appserv/was/WAS_V7_64-bit_performance.pdf

 With them the total amount of heap memory decreases. They don't slow down the
class usage significantly, and the memory reduction reduces the CPU cache
pressure, increasing the performance a bit.

 Will this idea be useful for 64 bit D/DMD too, for D GC-managed class
references (not for raw pointers)?

 Bye,
 bearophile

This probably can't work for D.

The problem is that D allows casting references to pointers. As we all 
know, a pointer is a distinct type from a reference, and casting a 
pointer to e.g. an integer type is perfectly valid and sometimes 
necessary. So, that is to say, this would blow up when doing the 
reference -> pointer conversion.

You could do it such that when one does the ref -> ptr conversion, the 
reference is simply expanded to a normal pointer, and vice versa. 
However, I'm not sure if this will hold water in the long run.

-- 
- Alex

deadalnix <deadalnix gmail.com> writes:

Le 12/03/2012 01:44, Alex Rønne Petersen a écrit :
 On 12-03-2012 01:41, bearophile wrote:
 A significant percentage of heap memory in a 64 bit JavaVM is used by
 references, that are 8 bytes long. To reduce this problem they have
 invented "compressed references" (useful only for 64 bit systems),
 that turn references to 32 bit again:

 https://blogs.oracle.com/jrockit/entry/understanding_compressed_refer

 ftp://public.dhe.ibm.com/software/webserver/appserv/was/WAS_V7_64-bit_performance.pdf


 With them the total amount of heap memory decreases. They don't slow
 down the class usage significantly, and the memory reduction reduces
 the CPU cache pressure, increasing the performance a bit.

 Will this idea be useful for 64 bit D/DMD too, for D GC-managed class
 references (not for raw pointers)?

 Bye,
 bearophile

 This probably can't work for D.

 The problem is that D allows casting references to pointers. As we all
 know, a pointer is a distinct type from a reference, and casting a
 pointer to e.g. an integer type is perfectly valid and sometimes
 necessary. So, that is to say, this would blow up when doing the
 reference -> pointer conversion.

 You could do it such that when one does the ref -> ptr conversion, the
 reference is simply expanded to a normal pointer, and vice versa.
 However, I'm not sure if this will hold water in the long run.

Additionally, 64bits pointer is good for us : 
http://www.deadalnix.me/2012/03/05/impact-of-64bits-vs-32bits-when-u
ing-non-precise-gc/ 
(shameless autopromotion inside).