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Message-ID: <3A89C304.89BD74A0@etr-usa.com>
Date: Tue, 13 Feb 2001 16:28:04 -0700
From: Warren Young <warren@etr-usa.com>
Organization: -ENOENT
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To: Cygwin-L <cygwin@cygwin.com>
Subject: Re: Optimizing away "ReadFile" calls when Make calls stat()
References: <4.3.1.2.20010213134821.019a7130@pop.ma.ultranet.com> <20010213190131.14369.qmail@lizard.curl.com> <200102131935.OAA09136@envy.delorie.com> <20010213194612.17311.qmail@lizard.curl.com>
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jik-cygwin@curl.com wrote:
> 
> As I've noted separately, reading tens of thousands of files even once
> incurs a significant performance penalty.  The change I've proposed
> can eliminate reading them at all.

Even stat() under Linux does at least one disk read.  You can't
completely optimize away disk I/O for stat().

The main culprit is that this is one of many places where Unix doesn't
map onto Win32 well at all.  The VC++ RTL doesn't use ReadFile() to
implement _stat() at all.  When it checks for things like stat.st_mode
== S_IEXEC, it simply checks the filename extension for .exe, .com or
.bat.  Cygwin can't do that -- it must look at the file's magic bytes to
see if it's an executable, or a #! style script.  

stat() on Unixen doesn't do either of these things; all the info stat()
reports is in the inode.  The info in a prototypical Unix inode is
scattered in many different places in Win32, which makes the emulator
for a call like stat() slow.

Maybe a better optimization strategy would be to patch GNU Make. 
Wherever it does a stat() to find the modification time, do something
like this:

        struct stat st;
#ifdef CYGWIN
        WIN32_FIND_DATA findinfo;
        HANDLE h = FindFirstFile(filename, &findinfo);
        st.st_mtime = findinfo.ftLastWriteTime;
        FindClose(h);
#else
        stat(filename, &st);
#endif

(ftLastWriteTime will probably need translation into a time_t, but
that's a small matter.)
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