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Message-ID: <3A89C304.89BD74A0@etr-usa.com>
Date: Tue, 13 Feb 2001 16:28:04 -0700
From: Warren Young <warren AT etr-usa DOT com>
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To: Cygwin-L <cygwin AT cygwin DOT com>
Subject: Re: Optimizing away "ReadFile" calls when Make calls stat()
References: <4 DOT 3 DOT 1 DOT 2 DOT 20010213134821 DOT 019a7130 AT pop DOT ma DOT ultranet DOT com> <20010213190131 DOT 14369 DOT qmail AT lizard DOT curl DOT com> <200102131935 DOT OAA09136 AT envy DOT delorie DOT com> <20010213194612 DOT 17311 DOT qmail AT lizard DOT curl DOT com>
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jik-cygwin AT curl DOT 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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