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MathGroup Archive 2001

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Re: Behaviour of Plot/ListPlot & Export

  • To: mathgroup at smc.vnet.net
  • Subject: [mg31764] Re: [mg31722] Behaviour of Plot/ListPlot & Export
  • From: SteveC <steve at fractalus.com>
  • Date: Sat, 1 Dec 2001 02:44:57 -0500 (EST)
  • References: <200111290513.AAA22173@smc.vnet.net> <8E77728D5EF5@ch1.irl.cri.nz>
  • Sender: owner-wri-mathgroup at wolfram.com

* Manfred Plagmann (m.plagmann at irl.cri.nz) wrote:
> 100. There is no bug, it is a feature!

Point taken - I assumed it would behave the same a Table :-) I was wrong.

> In your case of slow image saving I suggest you save each image right after 
> it is created as I have never noted any delays when I had just a few images 

Ok lets quantify this and clear up a few things:

My machine has 1 Gigabyte of memory, it pretty much *never* uses the swap
partition.

The CPU usage during an export of an animated gif is most of the time 0%. 
Literally. Every now and then the cpu usage will go to something like 
this:

  PID USER     PRI  NI  SIZE  RSS SHARE STAT  LIB %CPU %MEM   TIME COMMAND
 5458 steve     15   0  5244 5244  2500 S       0  7.9  0.5   0:06 
MathKernel
 5450 steve     12   0  5764 5764  3112 S       0  6.9  0.6   0:08 
Mathematica
 5460 steve     11   0   448  448   292 S       0  2.9  0.0   0:02 PNM.exe
 5402 steve     11   0  1348 1348  1140 S       0  1.5  0.1   0:21 mpg123

As you can see, playing an mp3 uses a comparable amount. Strange huh?

Now, Lets export one image:

Timing[Export["a.gif", Table[
      Show[Graphics[Line[{{0, 0}, {1, 1}}]]], {x, 0, 100}]]
  ]

Result:

{0.14 Second, "a.jpg"}

Now lets time exporting 100:

Timing[Export["a.gif", Table[
      Show[Graphics[Line[{{0, 0}, {1, 1}}]]], {x, 0, 100}]]
  ]

{9.11 Second, "a.gif"}

This is what you'd expect, but in fact it takes a *lot* longer:

a = Date[];
Timing[Export["a.gif", Table[
        Show[Graphics[Line[{{0, 0}, {1, 1}}]]], {x, 0, 100}]]
    ];
Date[] - a

result:

{0, 0, 0, 0, 3, -1}

So lets say on the order of 3 minutes. Thats a bit bigger than 10 seconds, 
and as noted most of that time is spent doing nothing. Anyway might as 
well spend some time running an strace on it.

Mathematica appears to spend a lot of time on these:
write(3, "\17\2\2\0\7\6\340\2", 8)      = 8
read(3, 0xbfffe6b0, 32)                 = -1 EAGAIN (Resource temporarily 
unavailable)

and

oldselect(1024, [], [], [17], {0, 0})   = 0 (Timeout)
write(17, "\\C", 2)                     = 2
oldselect(1024, [18], [], [], {0, 0})   = 1 (in [18], left {0, 0})
oldselect(1024, [18], [], [], {0, 0})   = 1 (in [18], left {0, 0})
read(18, "\\c", 1024)                   = 2
oldselect(1024, [18], [], [], {0, 0})   = 0 (Timeout)
oldselect(1024, [18], [], [], {0, 0})   = 0 (Timeout)

so something is timing out on its IO, which I guess is taking up all the 
time. So my question now is, whats it timeing out on and why?

There is nothing of interest in any system logs, and disk access has never 
been a problem.

The full strace is at http://www.fractalus.com/steve/tmp/tmp2.bz2 (450k)

I accept my comments on storing up a table of images is wrong, but I would 
be interested in seeing it not doing that. Granted large tables of images 
should be wrung through the Gimp or ImageMagick.

have fun,

SteveC steve at fractalus.com fractalus.com/steve


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