PHP elapsed time changes after certain time [closed] - php

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I have calculated the elapsed time of two process in my program. When I execute the program the elapse time in 0.203 sec but if I leave the page for a while the elapse time changes and become 0.173. What is the reaosn for this? My php program is
include ('db.php');
$data=array();
session_start();
$start_time= microtime(true);
if (isset($_SESSION['img']))
{
$image=$_SESSION['img'];
$addr="C:/Users/adithi.a/Desktop/FashionSearch/trial/db_features/distrib/db_features.exe $image";
exec($addr,$data);
/*for($i=18;$i<=34;$i++)
{
if($i!=30)
{
echo $data[$i]."<br>";
}
}*/
$start_time1=microtime(true);
$result=mysql_query("select tbl_features.img_id,img_path,((pow(($data[18]-features_1),2))+(pow(($data[19]-features_2),2))+(pow(($data[20]-features_3),2))+(pow(($data[21]-features_4),2))+(pow(($data[22]-features_5),2))+(pow(($data[23]-features_6),2))+(pow(($data[24]-features_7),2))+(pow(($data[25]-features_8),2))+(pow(($data[26]-features_9),2))+(pow(($data[27]-features_10),2))+(pow(($data[28]-features_11),2))+(pow(($data[29]-features_12),2))+(pow(($data[31]-features_13),2))+(pow(($data[32]-features_14),2))+(pow(($data[33]-features_15),2))+(pow(($data[34]-features_16),2))) as distance from tbl_features join tbl_image where tbl_features.img_id=tbl_image.img_id AND tbl_features.img_id>=92303 AND tbl_features.img_id<124232 ORDER BY distance ASC LIMIT 6") or die(mysql_error());
while($num=mysql_fetch_assoc($result))
{
echo "<a href='Dressinformation.php?image=$num[img_id]'><div class='imgdiv'><img src='$num[img_path]'></div></a>";
//echo $num["img_id"]." ".$num["img_path"]." ".$num["distance"]."<br>";
}
$stop_time1= microtime(true);
$time1=$stop_time1-$start_time1;
print "Euclidean distance time is $time1 seconds";
$stop_time= microtime(true);
$time=$stop_time-$start_time;
print "elapse time was $time seconds.";
}
else
{
echo "Please upload image";
}

Answer 1

There are tens of factors envolved. Just listing some:
If the HD is busy it will affect how long your program takes to load. On Windows it usually is, sinse its constantly doing ACL queries for reasons beyond me. Even if most of the stuff going on is cached in some form, PHP alone don't cache scripts and will read and parse it every time.
Your MySQL Server is probably running in Dev Mode, which decreases its responsiveness and memory usage to prioritize the user's processes. It also may or may not have the rows you are trying to SELECT in cache, and of course when we are talking about milliseconds it can make a lot of difference. Running the same query several times may cause MySQL to make it available in cache, what can explain why it becomes faster after a while.
Sinse Windows XP there is the Prefetcher, a technology that keeps a record of what your program needs to load so it will load faster next time you run it. If you ever noticed how a program is so much slower when you first run it on your machine, you now see why. From the second run on, it will boost up thanks to the Prefetcher. But as far as I know it does no further improvements on each subsequent run so its probably of little effect here.
And sinse Windows Vista, every Windows version also comes with SuperFetch, a technology that keeps commonly used programs pre-loaded in the "non-used" part of the RAM, and if yours was lucky enough to be elected it may explain why this boost happened. Thats also why from Windows Vista on the machine will perform so much better with larger amounts of RAM, even if you don't use it all.