Mail Archives: djgpp/1999/09/30/18:56:21

From: skb AT xmission DOT removethis DOT com (Scott Brown)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Reading directories, readdir/stat too slow
Date: Thu, 30 Sep 1999 22:34:47 GMT
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On Thu, 30 Sep 1999 16:17:21 +0200, Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>

>On Thu, 30 Sep 1999, Scott Brown wrote:
>> Unfortunately, I find that opendir/readdir, when combined with a stat
>> call (to get file mode/size/etc) is a *lot* slower than findfirst,
>> taking on the order of 70-100 times longer to perform the same work.
>> When running against tens of thousands of files in hundreds of
>> directories, it is a significant problem.
>stat is expensive (it isn't easy to get all that info on DOS; you won't 
>believe how closely DOS guards some of its dirty secrets ;-).  But 100 
>times slower seems to be too much; I suspect your system is not set up in 
>an optimal way.  See section 3.9 of the FAQ; in particular, make sure you 
>have a disk cache installed.

Well, I'm running under Windows 95, which has a built-in disk cache,
and my hardware is plenty powerful for this kind of work (K6-200
w/64Mb).  Statistics from sysmon are limited, but it shows that my
disk cache hasn't dropped below 2.5Mb in the last little while.

>A 10-fold slow-down when using stat is 
>something I would expect, but not 100-fold.

It seemed pretty ridiculous to me as well.  I put together a simple
timed test and ran it against a set of about 30,000 files in 350
directories; I ran each test twice and took the second result, to give
the OS a fair chance to load the cache.

The findfirst test finished in 1.98 seconds, while the readdir/stat
test finished in a whopping 133.96 seconds.  I believe my test code is
fair, but second opinions are welcome; grab a copy here:

>Read the docs ;-).

That always helps...

>No, seriously: the documentation of _djstat_flags in describes 
>several flags that can be set to disable computing some expensive 
>members of struct stat for which you don't have any use.  For the 
>fastest operation, you should disable all features but those which your 
>application needs.  Doing so is known to speed up stat tremendously.

I only need the mode, and the size and timestamp for files.  After I
R'd TFM, I tried setting *all* of the _STAT... bits, but the results
were disappointing; certainly not what I'd term a "trememdous"
improvement.  Instead of taking 133.96 seconds to finish, the test
took only 125.44 seconds.

My test program is fairly representative of the kind of directory
traversal code I use in my applications.  Could there be something
else that I am overlooking?

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