Cygwin fstat and NT caching

[Dan Morris <dmorris at tiqit dot com>]

My supervisor pointed out to me yesterday that "du" takes a long time to run
under Cygwin; out of curiosity, I've been looking into the source of the
delays, and I'm curious if anyone else has any insights... 

This is me running 'du' on my entire hard drive under cygwin :

dmorris@SOAPY /edrive
$ time du

... blah blah blah ... all my files go by ... disk spins like mad ... 

... I get some coffee ...

0       ./cygwin/edrive
1       ./cygwin/.ssh
208758  ./cygwin
7336071 .

real    5m35.523s
user    0m10.064s
sys     0m32.877s

Five minutes is a long time.

Now this is me running the Windy Tree CommandPack's version of "du" on the
same drive, from inside Cygwin (so I can use "time") :

dmorris@SOAPY /edrive
$ time /edrive/Program\ Files/utils/commands/du.exe

Windy Tree CommandPack - du (diskusage)
(C) Copyright 1999 by Windy Tree. More info at www.windytree.com
-------------------------------------------------------------------------------

... files race by quickly ... disk doesn't move ... clearly using cached
inodes...

   4775838      e:\cygwin\home
         0      e:\cygwin\edrive
       350      e:\cygwin\.ssh
 209518621      e:\cygwin

7494592864 Bytes total in 67845 files, 4516 directories

real    0m35.301s
user    0m0.170s
sys     0m0.120s


Thirty seconds for WindyTree, five minutes for Cygwin; this result is
repeatable (that is, the result you see above doesn't mean that I'm just
running the Cygwin version first, then letting the WindyTree version take
advantage of all the caching).

Anyone know why this might be?  I looked through the du code; the only
repeated filesystem access seems to be through stat() and lstat().  So I
looked through stat() and lstat(), and I can't see any place where disk
flushes are explicitly forced, or any place where Win32 calls are made with
specific flags for ignoring the inode cache ... these seem to be the Win32
filesystem calls that get made :

GetVolumeInformation
GetDriveType
GetFileInformationByHandle
GetFileSize
GetFileType
FindFirstFile
 
I don't know why any of them would ignore the cache... any ideas?  I'm
running Win2000, if that helps anyone...

Thanks...

-Dan

Dan Morris
http://techhouse.brown.edu/dmorris

Tiqit Computers
http://www.tiqit.com


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