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From: tps@chem.ucsd.edu (Tom Stockfisch)
Newsgroups: comp.lang.c
Subject: Re: Memory Allocation
Message-ID: <467@chem.ucsd.EDU>
Date: 9 May 89 06:19:31 GMT
References: <10946@bloom-beacon.MIT.EDU> <12546@ut-emx.UUCP> <29978@apple.Apple.COM> <17252@mimsy.UUCP> <1989May3.201118.10221@utzoo.uucp> <10202@smoke.BRL.MIL> <461@chem.ucsd.EDU> <10206@smoke.BRL.MIL>
Reply-To: tps@chem.ucsd.edu (Tom Stockfisch)
Organization: Chemistry Dept, UC San Diego
Lines: 19

In article <10206@smoke.BRL.MIL> gwyn@brl.arpa (Doug Gwyn) writes:
>In article <461@chem.ucsd.EDU> I (Tom Stockfisch) write:
>>You waste up to a factor of 4 with a power-of-2 implementation,
>>not including fragmentation.

>..., I see a factor of two (not four) here,...

Here is the problem, at least with our BSD malloc():  I am reading in data
that needs to be put in one contiguous array.  I call malloc() with a certain
amount, fill that up, call realloc(), fill that up, etc.  Every time
that I cross a power of two boundary, realloc() puts my old memory on the
free list for the previous power of two and gets a whole new chunk.  Thus,
for a large final array size of n = (2^m) + 1 it has allocated
2^4 + 2^5 + 2^6 + 2^7 + 2^8 + ... + 2^(m+1), which is nearly 2^(m+2), which
is nearly 4*n.

-- 

|| Tom Stockfisch, UCSD Chemistry	tps@chem.ucsd.edu