2008.08.19 05:17 "[Tiff] Regarding DICONDE and its Specification", by Harsha

2008.08.22 15:45 "Re: [Tiff] creating sparse files......", by Rogier Wolff

On Fri, Aug 22, 2008 at 10:11:42AM -0300, Toby Thain wrote:

static int isallzero (tdata_t buf, tsize_t size)
        int i;
        for (i=0;i<size;i++)
                if (buf[i]) return 0;
        return 1;

This appears unnecessarily inefficient. It's going to be much cheaper to test whole longwords (or whatever data alignment will allow) than byte by byte. (I wonder if something clever can be done with MMX/SSE on newer chips?)

Yes, this is the "quick and dirty" implementation of this function. The advantage of doing it this simple is that it will ALWAYS work. No stupid bugs that if the only nonzero byte is in the partial long just beyond the

Keep in mind that when this runs, the CPU will be getting whole cache lines from memory. When the cache has been filled, the CPU runs at say 3GHz. Or three cycles per ns. It will likely run the loop three times a nanosecond as well. The memory fetching that needs to go on, is likely to cost on the order of tens of nanoseconds per cache line. DDR still runs at 200 MHz, and you have a two cycle latency for reads, right? That's around 10 ns. At that point you can START to recieve data from memory. The whole cache line then takes a few more cycles....

Anway, my philosophy is always FIRST get it to work, optimize later.

static tsize_t
_tiffWriteProc(thandle_t fd, tdata_t buf, tsize_t size)
        if (isallzero (buf, size))
                return (lseek ((int) fd, (off_t)size, SEEK_CUR));
                return ((tsize_t) write((int) fd, buf, (size_t) size));

This penalises all writes to optimise a very special case. Not saying it can't be rationalised, but pros and cons can be debated.

And you're calling this an optimization. And I agree. However, in my case saving a factor of 21 on disk space is i'd say "worth it".

I work in datarecovery. I modified our "read-the-data-from-the-disk" program to check for zeroes. Of course it got the commandline flag that allowed me to turn it off for disks that didn't have many zeroes, so that things would go faster. Bad move. Or at least unneccesary in hindsight. Performance is NOT influenced by the check for zeroes, and the gain by not writing the zeroes is always noticable.

But, yes, officially it could cost you some CPU cycles, which are not returned by saving on writes. (checking for zeroes and seeking is much more efficient than just doing the write! It not only saves you disk space, but also CPU cycles!)



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