Thank you for the answer, That explains why loading a bitmap (without the shadow byte-string) didn't raise the the value reported by current-memory-use.
I'm wondering now in what way this helps the GC and the memory-limit checks? The reason I started looking into this was that DrRacket kept running out of memory and closing down without a warning (even with a memory limit set to 512 MB). So to me it seems that for current-memory-use and DrRacket's memory limit check this trick doesn't really work. But maybe this is not the intended purpose of the shadow byte-string. Bert > From: mfl...@cs.utah.edu > To: bed...@hotmail.com > CC: users@racket-lang.org > Subject: Re: [racket] bitmap% size in memory > Date: Wed, 19 Dec 2012 07:11:24 -0700 > > The "shadow" byte string is indeed unused. It's there only as an > accounting trick for Racket's garbage collector. > > The memory used for the actual bitmap is not visible to the garbage > collector, so it doesn't directly count towards a program's (or > custodian's) memory use. The shadow byte string is allocated and kept > alive during the same time as an actual bitmap to create GC-visible > memory use. That is, the shadow byte string triggers full garbage > collections and memory-limit checks as needed to keep the system as a > whole working right. > > This trick does use about twice the memory that a bitmap would > otherwise use, but it makes the garbage collector and bitmaps play > nicely without complex changes to the garbage collector. > > At Wed, 19 Dec 2012 08:10:37 +0100, Bert De Ketelaere wrote: > > > > Hello all, > > > > When loading large pictures (± 4300x2600 pixels) I would expect that they > > takes > > up around 45Mb of memory. > > But when I monitored my program in windows task-manager I noticed it was > > more > > in the neighborhood of 100Mb (after garbage collection, the in-between > > values > > goes over 200Mb) > > Hoping to find a reason for this I went looking into racket/draw/bitmap and > > noticed that there is a variable "shadow" that is set to a zeroed out > > byte-string, the size of the picture. But the only thing happening with > > this > > "shadow" is it's definition and a set!. It is never read. > > I tried removing the two references to "shadow", and in my programs > > everythings > > seems to keep working, with the added bonus that the picture now indeed > > takes > > only around 45Mb instead of 100Mb. > > > > two questions: > > As far as I understand classes, since shadow is locally defined, no parent > > or > > child can access shadow, so removing it should be ok. Is this right? > > Is there a way to see how much memory an object occupies? > > > > Bert > > > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users
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