Search slightly undercounts when matching many tiny frames

[brendangregg]

I'll create an extreme example to highlight it, by creating an artificial folded file with 100 tiny frames:

for i in `seq 1 100`; do echo "big_a;big_b;big_c;big_d;big_e;big_f;big_g;tiny_h$i 1"; done > search_test.txt
echo "big_x;big_y;big_z 100" >> search_test.txt
./flamegraph.pl < search_test.txt > search_test.svg

If you Ctrl-F search for "big_g", it matches 50%. As it should.

But if you Ctrl-F search for "tiny", it matches 45.5%. We've lost 0.045% for each matched frame.

This sounds like a rounding issue in the JavaScript search code. These are adjacent in the svg:

<g class="func_g" onmouseover="s(this)" onmouseout="c()" onclick="zoom(this)">
<title>tiny_h11 (1 samples, 0.50%)</title><rect x="27.7" y="33" width="5.9" height="15.0" fill="rgb(248,191,42)" rx="2" ry="2" />
<text text-anchor="" x="30.70" y="43.5" font-size="12" font-family="Verdana" fill="rgb(0,0,0)"  ></text>
</g>
<g class="func_g" onmouseover="s(this)" onmouseout="c()" onclick="zoom(this)">
<title>tiny_h12 (1 samples, 0.50%)</title><rect x="33.6" y="33" width="5.9" height="15.0" fill="rgb(230,70,1)" rx="2" ry="2" />
<text text-anchor="" x="36.60" y="43.5" font-size="12" font-family="Verdana" fill="rgb(0,0,0)"  ></text>
</g>
<g class="func_g" onmouseover="s(this)" onmouseout="c()" onclick="zoom(this)">
<title>tiny_h13 (1 samples, 0.50%)</title><rect x="39.5" y="33" width="5.9" height="15.0" fill="rgb(215,130,36)" rx="2" ry="2" />
<text text-anchor="" x="42.50" y="43.5" font-size="12" font-family="Verdana" fill="rgb(0,0,0)"  ></text>
</g>

So the "x" and "width" variables show that these's no space between these frames. It's the widths that are summed to get the total percent, and the size of the "big" frames is 590, so 100 x 5.9 == 590. So this approach should work, but we're losing a tiny bit. This tells me that the problem is not the Perl code, but the JavaScript code that's adding the widths. (JavaScript floating point?)

[cwarden]

Yep, looks like another floating point problem.

Work-around it like in zoom?

[brendangregg]

thanks. I'm very glad that variable is called "fudge" -- makes it easy to remember what it's for!