Re: Cygwin Performance and stat()

[Christian Franke <Christian dot Franke at t-online dot de>]

Christopher Faylor wrote:
> On Sun, May 30, 2010 at 12:51:31PM -0700, Christopher Wingert wrote:
>    
> > I assume POSIX compatibility.  However, I bet there are cases where one
> > can sacrifice compatibility for performance (configurable with an
> > environment flag of course).
> >
> >      

The problem is that POSIX stat() requires to provide all information in 
struct stat. This is expensive: convert ACL into mode, convert filetimes 
into time_t, lookup uids, invent ino, ....
Typically applications don't need all info but there is no way to tell 
this to stat().


> > See
> >
> > http://marc.info/?l=git&m=122278284210941
> >
> > for an example.
> >      
> Yes, I got what you meant.  I think that bypassing Cygwin, assuming you
> can do something good enough is a remarkably bad idea.
>
>    

A probably better idea would be to add another stat() variant (this is 
IMO missing in POSIX) to Cygwin which allows to specify which info is 
needed.

Something like:

enum {
  CYGSTAT_MODE = 0x01,
  CYGSTAT_INO = 0x02,
  CYGSTAT_SIZE = 0x04,
  CYGSTAT_ATIME = 0x08,
  ...
};

int cygwin_stat4(const char *path, struct stat *buf, unsigned needed, 
unsigned *pvalid);

Where 'needed' contains all CYGSTAT_* flags for the stat fields needed. 
If 'pvalid' is != 0, the flags for the valid fields are returned in the 
variable. This can be a superset of 'needed' if some info is available 
for free.

Usage example:

// Get st_size only.
#ifdef __CYGWIN__
#define stat_size(p, b) cygwin_stat4(p, b, CYGSTAT_SIZE, NULL)
#else
#define stat_size(p, b) stat(p, b)
#endif

Christian


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