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Date: Fri, 29 Jul 2005 10:09:59 -0700
From: Yitzchak Scott-Thoennes <sthoenna AT efn DOT org>
To: cygwin AT cygwin DOT com
Subject: Re: heap_chunk_in_mb default value (Was Re: perl - segfault on "free  unused scalar")
Message-ID: <20050729170958.GA872@efn.org>
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On Wed, Jul 27, 2005 at 05:07:23PM -0700, Stephan Mueller wrote:
> "Igor Pechtchanski wrote:
> "
> " On Thu, 28 Jul 2005, Krzysztof Duleba wrote:
> " > > > I've simplified the test case. It seems that Cygwin perl can't
> " > > > handle too much memory. For instance:
> " > > >
> " > > > $ perl -e '$a="a"x(200 * 1024 * 1024); sleep 9'
> " > > >
> " > > > OK, this could have failed because $a might require 200 MB of
> " > > > continuous space.
> " > >
> " > > Actually, $a requires *more* than 200MB of continuous space.  Perl
> " > > characters are 2 bytes, so you're allocating at least 400MB of
> space!
> " >
> " > Right, UTF. I completely forgot about that.
> " 
> " Unicode, actually.
> 
> Unicode is a standard that defines 'code points' (numeric values) for a
> whole lot of different characters.  UTF-8 is a specific encoding of
> Unicode.  It has the nifty property that ASCII characters are encoded
> just as in ASCII -- one byte, with the high bit clear, and the low seven
> bits representing a character in the range 0..127.  Characters above the
> ASCII range require multiple bytes -- sometimes two, sometimes more.
> The algorithm is quite clever; find it in The Unicode Standard or with a
> quick Google search.
> 
> Another popular encoding is UCS-2, which is roughly "16-bit words each
> holding one Unicode character".
> 
> The latter is frequently what people think of as "Unicode".  The former
> is what perl uses internally to encode characters.
> 
> End result is that the perl internal representation in the example above
> probably only needs about 200MB of space, and not double that, as
> suggested.

Correct; perl uses UTF-8 (actually, an extension of UTF-8 which allows
codepoints up to 2**72-1).

However code like the above does end up using twice the space; it's
allocated once to store the result of the x operation and again when
it's copied to $a.

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