Re: PATCH: fail to improve psymtab memory consumption

[Andrew Cagney <ac131313@cygnus.com>]

>>> I actually use mmap when possible, but that's for speed, rather than
>>> memory savings.
> 
>> 
>> Is it really faster?
> 
> Yes, because it saves buffer copying (I think, it's 2am, which is
> about the time i usually say something stupid, so feel free to smack
> me around if i'm wrong)
> (You don't have to dandy about copying buffers into the right
> place. With an mmap'd area, it's already *in* the right place.)
> 
> 
> Nathan myers confirms my suspicion, as well.
> 
> http://pgsql.profnet.pl/mhonarc/pgsql-hackers/2001-05/msg01233.html
> "
> Using mmap() in place of disk read() almost always results in enough
> performance improvement to make doing so worth a lot of disruption."
> "


Does this require serial or random file i/o?


> Everything i can pull up on google says mmap is so much faster than
> malloc+fread that it's not even funny, which means it's not just that
> it feels faster.  On a 100 meg debug info file, you can easily notice
> the difference.


What the heck.  I'll think out loud.

The dwarf2 sections are parsed serially, correct?  That means either:

	o	the entire section is read in (a large
		slow file cache thrashing copy) and then
		parse that buffer

	o	the section is mmap'd - gdb's memory image
		grows by a large amount

	o	the section is read, bit by bit, on demand
		using FILE and the hosts buffer cache.
		Some OS's even implement this as a mmap().

I don't know but back in the good old days CPU's and disks were vaguely 
comparable.  How a days, the disks are the same speed but the CPU's are 
fasater.  Simple FILE I/O could prove faster than you're expecting.

	Andrew