Poor read() performance, and I can't profile it

Chuck Swiger cswiger at mac.com
Thu Jun 12 03:20:19 UTC 2008

On Jun 11, 2008, at 12:42 PM, Kirk Strauser wrote:
> I'm almost ready to give up on this.  I've gone as far as completely  
> rewriting the
> original C++ program into straightforward C, and still the  
> performance is terrible on
> FreeBSD versus Linux.

On Linux, GNU libc buffers file data much more extensively than  
FreeBSD's libc does.  It means that doing things like reading a dozen  
bytes or so at a time is not intolerably slow on the former system,  
but that doesn't mean that it's a great idea either.

If your data files are small enough to fit into 2GB of address space,  
try using mmap() and then treat the file(s) as an array of records or  
memoblocks or whatever, and let the VM system deal with paging in the  
parts of the file you need.  Otherwise, don't fread() 1 record at a  
time, read in at least a (VM page / sizeof(record)) number of records  
at a time into a bigger buffer, and then process that in RAM rather  
than trying to fseek in little increments.

(This is the opposite of calling setvbuf() to set the I/O buffer to,  
say, 13 bytes...)

Also, if you're malloc'ing and freeing buf & memohead with every  
iteration of the loop, you're just thrashing the malloc system;  
instead, allocate your buffers once before the loop, and reuse them  
(zeroize or copy new data over the previous results) instead.


> Also note that on the FreeBSD machine, I have enough RAM that to  
> buffer the entire
> file, and in practice gstat shows that the drives are idle for  
> subsequent runs after
> the first one.
> Right now my code looks a lot like:
>   for(recordnum = 0; recordnum < recordcount; recordnum++) {
> 	buf = malloc(recordlength);
> 	fread(buf, recordlength, 1, dbffile);
>        /* Do stuff with buf */
>        memoblock = getmemoblock(buf);
>        /* Skip to the requested block if we're not already there */
> 	if(memoblock != currentmemofileblock) {
> 	    currentmemofileblock = memoblock;
> 	    fseek(memofile, currentmemofileblock * memoblocksize, SEEK_SET);
> 	}
> 	memohead = malloc(memoblocksize);
> 	fread(memohead, memoblocksize, 1, memofile);
> 	currentmemofileblock++;
>        /* Do stuff with memohead */
>        free(memohead);
> 	free(buf);
>    }
> ...where recordlength == 13 in this one case.  Given that the whole  
> file is buffered in
> RAM, the small reads shouldn't make a difference, should they?  I've  
> played with
> setvbuf() and it shaves off a few percent of runtime, but nothing to  
> write home about.
> Now, memofile gets quite a lot of seeks.  Again, that shouldn't make  
> too much of a
> difference if it's already buffered in RAM, should it?  setvbuf() on  
> that file that
> gets lots of random access actually made performance worse.
> What else can I do to make my code run as well on FreeBSD as it does  
> on a much wimpier
> Linux machine?  I'm almost to the point of throwing in the towel and  
> making a Linux
> server to do nothing more than run this one program if I can't  
> FreeBSD's performance
> more on parity, and I honestly never thought I'd be considering that.
> I'll gladly give shell access with my code and sample data files if  
> anyone is
> interested in testing it.
> -- 
> Kirk Strauser
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