Speed of read vs. fread

[Spencer W. Thomas]

In article <626 at ihlts.UUCP> lied at ihlts.UUCP (Bob Lied) writes:
>I've been told that fread/fwrite is faster because
>it buffers.  On the other hand, I've been told that
>read/write is faster because (a) read always does
>a one block read-ahead if it can and (b) it avoids
>the overhead of the fread/fwrite abstraction.
>
>Given a choice, and doing I/O in chunks instead of
>characters, which should I use for performance?
>Please explain your selection, and no lectures on
>tuning too early, OK?
>
Well, I did a simple experiment, contrasting read and write vs fread and
fwrite performance on a 4.2a (or is it 4.3) system.  This has the fread
and fwrite enhancements (the idea for which was taken from System V, I
believe, so results should be comparable).  My experiment, with results
tabulated below, shows that fread and fwrite are MUCH better for small
record sizes, and are worse for large record sizes (but not too much).
These tests were run on a basically unloaded VAX 750 running the
enhanced 4.2bsd system (which may be called 4.3 someday, I dunno).  The
file system block size for the first test was 8192/1024 and for the
second it was 4096/512.  (For those of you not familiar with the 4.2
file system, the first number is the file system block size, and
the 2nd is the fragment size.  For files this size, you can mostly
ignore the fragment size.)

Reading /vmunix (274432 bytes) with

record	|	    read		    	    fread
size  	|user	system	clock	%	|user	system	clock	%
--------+-------------------------------+-------------------------------
1	|17.4	499.7	9:35	89%	|40.0	1.5	1:10	68%
512	|0.0	1.4	0:02	74%	|0.2	0.4	0:01	47%
8192	|0.0	0.4	0:01	41%	|0.2	0.5	0:01	54%

Writing a 1Mb file with

record	|	    write		    	    fwrite
size  	|user	system	clock	%	|user	system	clock	%
--------+-------------------------------+-------------------------------
28	|3.3	134.5	2:34	89%	|5.6	3.6	0:10	88%
1024	|0.1	6.5	0:07	88%	|0.7	3.6	0:05	77%
8192	|0.0	3.4	0:04	77%	|0.8	3.3	0:04	89%

P.S., this also shows the performance of the 4.2 filesystem -- note that
I can read a 270Kb file in about 1 second, and can write a 1Mb file in
about 4 seconds.  Not too shabby!
-- 
=Spencer
	({ihnp4,decvax}!utah-cs!thomas, thomas at utah-cs.ARPA)
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