Hi folks. I was just trying to understand this optimization of the Compact Classes. If I understood correctly, most objects needs a second word there they have 30 bits of OOP of their class and 2 for the header type.
There are classes that have a LOT of instances. For those cases we have compact classes where their instances have a number using those 5 bits in the first word of the header, and thus, they don't need a second header. Then, less memory is used.
So...I understood ok ?
Now, if that is the situation. I have evaluated Smalltalk compactClassesArray and I see that in the array there are a lot of nils. 16 classes in Squeak and 15 in Pharo. The rest of the 31, are just nil.
Now I wonder if it is worth to do anything with that :
a) Use 4 bits instead of 5 and free one more bit in the OH b) remove completely CompactClasses as Adrian said c) Let 5 bits but then add more classes (up to 31) so that we can get benefits of that. Do you know if there are side effects of becomingCompact a class ?
If I evaluate SpaceTally new printSpaceAnalysis I get something like this:
Class code space # instances inst space percent ByteString 2217 71317 3621735 19.3 CompiledMethod 21064 56057 3517772 18.7 Bitmap 3893 2256 3424604 18.2 Array 2478 71108 2198420 11.7 ByteSymbol 959 37896 962465 5.1 ByteArray 2778 317 778297 4.1 MethodDictionary 1819 5964 449252 2.4 MCVersionInfo 1313 10369 414760 2.2 DateAndTime 6974 13793 331032 1.8 WeakArray 827 286 326900 1.7 Association 749 26784 321408 1.7 UUID 1528 10370 248880 1.3 Float 11442 16450 197400 1.1 ClassOrganizer 1398 5964 190848 1.0 ODatedEntry 263 7049 169176 0.9 Time 5530 10369 165904 0.9 Duration 3025 10290 164640 0.9 Date 3167 10289 164624 0.9 LargePositiveInteger 2515 14340 118169 0.6 Metaclass 4250 2876 115040 0.6 Point 6404 8476 101712 0.5 GlyphForm 307 2030 73080 0.4 FreeTypeCacheEntry 536 2238 71616 0.4 SpaceTallyItem 651 2509 60216 0.3 OrderedCollection 5351 2831 56620 0.3 FreeTypeFileInfo 797 321 30816 0.2 FreeTypeFontFamilyMember 1274 537 25776 0.1 Color 21202 1255 25100 0.1 TraitComposition 2651 2013 24156 0.1 Set 3510 1358 21728 0.1 Dictionary 5742 1344 21504 0.1 RemoteString 1533 1236 19776 0.1 SparseLargeTable 2376 2 18708 0.1 WordArray 1821 30 18608 0.1 SoundBuffer 3299 3 18508 0.1 Pragma 1558 862 17240 0.1 Preference 1534 379 16676 0.1 EventHandler 3216 125 14500 0.1 MorphExtension 3249 277 14404 0.1 Character 6428 939 11268 0.1 SortedCollection 1496 462 11088 0.1 AdditionalMethodState 2576 859 10304 0.1 ISOLanguageDefinition 18114 402 9648 0.1 TimeStamp 861 392 9408 0.1
So...for this quick evaluation, maybe we can become compact also the following classes for example:
Smalltalk compactClassesArray
#( ByteSymbol ByteArray MethodDictionary "Association" "WeakArray" "Metaclass" DateAndTime UUID Time Duration Date "MethodDictionary" ClassOrganizer Metaclass Pragma OrderedCollection RemoteString WordArray Color Character ) do: [:each | (Smalltalk at: each) becomeCompact]
Do this make sense ? How could I measure this? I took a Pharo image and compare the memory used by the VM running all test before and after those new compact classes. From the "Activity monitor" I noticed between 3 and 5 MB less used with these new compact classes.
Thanks in advance
Mariano
On Tue, May 4, 2010 at 2:12 PM, Adrian Lienhard adi@netstyle.ch wrote:
Hi Mariano,
I see two solutions
- make the identityHash (even) smaller
- remove compact classes
The former would probably not be hard to implement (just change some bit masks and then rehash all sets). I've brought up the idea about the latter some time ago on this mailing list. The idea is to remove compact classes to get a larger identityHash (trading memory against speed). After the removal, only two header types are left and hence you gain a spare bit. I think this shouldn't be too much work either, but I haven't come around to implementing it (yet).
Good luck ;) Adrian
On May 4, 2010, at 13:13 , Mariano Martinez Peck wrote:
Now...the question is, is there another free bit in the object header ?
I
need another one :(
Based on the comment at the bottom of Interpreter>>internalIsImmutable: I suspect that somebody else has their eye on that last available bit also ;-)
Yes, I know :( But for the moment I don't care to "integrate" my stuff.
I
just want to play and experiment. And even for that, I need 2 bits :( Of course, if I once make something good to integrate, I will have double problem :(
So...no solution ?