Lighttpd

Hi,
Here is my set up:
OS: linux
lighttpd ver: latest 1.4.31
client browser: firefox 13.0.1
configuration is shown as below:
---------------------------------------------------------------
server.document-root = "/var/www/html/"
server.port = 80
#server.username = "lighttpd"
#server.groupname = "lighttpd"

#server.username = "root"
#server.groupname = "root"

#server.bind = "192.168.2.161"
#server.tag ="lighttpd"

server.use-ipv6 = "enable"

server.errorlog = "/var/log/lighttpd/error.log"
accesslog.filename = "/var/log/lighttpd/access.log"

server.modules = (
"mod_access",
"mod_accesslog",
"mod_fastcgi",
"mod_rewrite",
"mod_auth",
"mod_ssi",

)

ssi.extension = ( ".html" )
fastcgi.debug = 1
fastcgi.server = ( ".php" => ((
"bin-path" => "/usr/local/bin/php-cgi",
"socket" => "/tmp/php.socket"
)))

1. mimetype mapping
   mimetype.assign = (
   ".pdf" => "application/pdf",
   ".sig" => "application/pgp-signature",
   ".spl" => "application/futuresplash",
   ".class" => "application/octet-stream",
   ".ps" => "application/postscript",
   ".torrent" => "application/x-bittorrent",
   ".dvi" => "application/x-dvi",
   ".gz" => "application/x-gzip",
   ".pac" => "application/x-ns-proxy-autoconfig",
   ".swf" => "application/x-shockwave-flash",
   ".tar.gz" => "application/x-tgz",
   ".tgz" => "application/x-tgz",
   ".tar" => "application/x-tar",
   ".zip" => "application/zip",
   ".mp3" => "audio/mpeg",
   ".m3u" => "audio/x-mpegurl",
   ".wma" => "audio/x-ms-wma",
   ".wax" => "audio/x-ms-wax",
   ".ogg" => "audio/x-wav",
   ".wav" => "audio/x-wav",
   ".gif" => "image/gif",
   ".jpg" => "image/jpeg",
   ".jpeg" => "image/jpeg",
   ".png" => "image/png",
   ".xbm" => "image/x-xbitmap",
   ".xpm" => "image/x-xpixmap",
   ".xwd" => "image/x-xwindowdump",
   ".css" => "text/css",
   ".html" => "text/html",
   ".htm" => "text/html",
   ".js" => "text/javascript",
   ".asc" => "text/plain",
   ".c" => "text/plain",
   ".conf" => "text/plain",
   ".text" => "text/plain",
   ".txt" => "text/plain",
   ".dtd" => "text/xml",
   ".xml" => "text/xml",
   ".mpeg" => "video/mpeg",
   ".mpg" => "video/mpeg",
   ".mov" => "video/quicktime",
   ".qt" => "video/quicktime",
   ".avi" => "video/x-msvideo",
   ".asf" => "video/x-ms-asf",
   ".asx" => "video/x-ms-asf",
   ".wmv" => "video/x-ms-wmv",
   ".bz2" => "application/x-bzip",
   ".tbz" => "application/x-bzip-compressed-tar",
   ".tar.bz2" => "application/x-bzip-compressed-tar"
   )
   index-file.names = ( "index.html", "index.php" )
   -------------------------------------------------------------------
   Issue:
   without doing anything but only periodically(every 10 sec) refresh the same web page(login.php),
   observe the available memory is gradually decreasing. (command: /bin/cat /proc/meminfo | /bin/grep MemFree:)
   as we can observe from the debug print-out,
   memory not always leaking in every refresh, but most of time it will decreases 28k as long as it gets decrease.
   we first suspect it has leak in mod_fastcgi.c,
   but according to the print-out, memory usage in the whole refreshment process is varying in many places else.
   Please kindly let us know how can we debug this issue and help investigate this problem.
   Step to reproduce:
   enable lighttpd with cfg above in server, connect to it and auto refresh the same page(any page),
   show memory usage to observe the memory info.
   Thank you very much for your kindly support