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Re: PATCH: fail to improve psymtab memory consumption
>>> 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