Message-Id: Comments: Authenticated sender is From: "Salvador Eduardo Tropea (SET)" Organization: INTI To: "John Machin" Date: Thu, 30 Oct 1997 13:58:34 +0000 MIME-Version: 1.0 Content-type: text/plain; charset=US-ASCII Content-transfer-encoding: 7BIT Subject: Re: malloc() CC: djgpp AT delorie DOT com Precedence: bulk "John Machin" wrote: > > "John Machin" wrote: > > > > > As mentioned in some discussions in this list, and in the > > > comments in malloc.c, the algorithm that is used allocates (or is > > > supposed to allocate) blocks of size 2^n. Of this, 4 bytes are > > > overhead. So a malloc(2044) -> 2048-byte block, malloc(2045) -> > > > 4096-byte block, etc. This means (approximately) an overhead of 0% > > > to 100% on the requested size, or a waste of 0% to 50% of the memory > > > grabbed. > > > > > > HOWEVER, experimental results, confirmed by reading the source, show > > > that the simplistic algorithm has been twiddled for request sizes > > > over 2044 bytes so that it skews the world by 4096 bytes. The > > > results are: > > [snipped values] > > You are missing something: If you have enough swap it doesn't matter > > because the memory that you allocate and don't touch just wastes > > virtual memory but is well remaped by the DPMI host so your % aren't > > true. Is hard to say the real waste. > > Oh. I thought if you allocated a page of memory (4096 bytes) and > touched only the first byte then it was all mapped ... what is the > unit of mapping??? Is 4K, but if you allocate 16384+overhead=16386 => 32768 => 8 pages only 5 will be touched and hence the wastage can be messured as 50%. > > > This could be fixed, but I am puzzled why DJ hasn't snarfed a better > > > malloc from somewhere -- I understand his comments about GNU GPL > > > that he made in a posting, but what about public-domain stuff??? > > Eli pointed why and I'll point an additional thing: > > DJGPP's malloc is HYPER fast. I know how it works (I even modified > > it) and I know how the Borland's one works (I have the sources) and > > beleive me, in a program that uses heavilly the heap the DJGPP's > > version is much, MUCH more faster. > > Speed is not the only consideration. 7 times! that's too much difference. > Robustness in the face of varied > patterns of malloc/free requests may be another consideration for > some ... Allocators which don't split/coalesce free blocks can break > down if the program allocates a large number of blocks of size x, > frees them, allocates/frees ditto sizes y,z, etc (where x,y,z etc > belong in different power-of-two buckets) --- unrealistic? No, e.g. > program == text editor, x == average len of source file line, y == > average len of record in data file1,... 1) Until now only a marginal number of persons experimented real problems with it. The fact is that this malloc is working for a long time. 2) My modified version of malloc (is in my site) lets work normally to malloc, but when all the memory is allocated it starts to split/join chuncks. I did it because I saw problems in my text editor. > > > Pardon me if this has been considered and rejected it for > > > reasons that I can't guess, but I'd suggest that "Doug Lea's malloc" > > > would be a good substitute. I got it off the web, whacked in a few > > > #defines, compiled it, and happiness prevailed; see below which is > > > the first few lines of the source file with my changes and his > > > "advertisement" and URLs. > > > > I got just a portion of the file. > > > > Like I said, "the first few lines of the source file". Ok, as you can see in my other mail I was able to recreate the file. Greetings, SET ------------------------------------ 0 -------------------------------- Visit my home page: http://www.geocities.com/SiliconValley/Vista/6552/ Salvador Eduardo Tropea (SET). (Electronics Engineer) Alternative e-mail: set-sot AT usa DOT net - ICQ: 2951574 Address: Curapaligue 2124, Caseros, 3 de Febrero Buenos Aires, (1678), ARGENTINA TE: +(541) 759 0013