Re: [PATCH] Don't do unneccesary memory copies in dd.

[Pádraig Brady]

Jim Meyering wrote:
> I was disappointed to see no performance improvement.
> But it's not too surprising: this affects dd only with a block
> size that's so low (512) that the cost of the memcpy is
> lost in the noise of the read/write syscalls.
> Maybe on some small/old CPU it actually does make a difference.

You could test the difference with the less common case
where both [io]bs are specified.

$ ./dd-before ibs=1M obs=1M if=/dev/zero of=/dev/null count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.100523 s, 1.0 GB/s

$ ./dd ibs=1M obs=1M if=/dev/zero of=/dev/null count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 0.188189 s, 5.3 GB/s

$ ./dd ibs=1M obs=512K if=/dev/zero of=/dev/null count=1000
1000+0 records in
2000+0 records out
1048576000 bytes (1.0 GB) copied, 0.635144 s, 1.7 GB/s

As an aside, I noticed that my fedora 8 system performance
got increasingly worse as the buffers increased beyond 512KiB?
This suggests that the read syscalls take longer with bigger ibs:

$ strace -e read -c ./dd ibs=1M obs=1M if=/dev/zero of=/dev/null count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 0.231632 s, 4.5 GB/s
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.037833          38      1005           read

$ strace -e read -c ./dd ibs=10M obs=10M if=/dev/zero of=/dev/null count=100
100+0 records in
100+0 records out
1048576000 bytes (1.0 GB) copied, 1.06246 s, 987 MB/s
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    1.030802        9817       105           read

While this suggests the dd process takes longer (user time)
with bigger obs:

$ time ./dd ibs=1M obs=512K if=/dev/zero of=/dev/null count=1000
1000+0 records in
2000+0 records out
1048576000 bytes (1.0 GB) copied, 0.601005 s, 1.7 GB/s

real    0m0.603s
user    0m0.375s
sys     0m0.228s
$ time ./dd ibs=1M obs=2M if=/dev/zero of=/dev/null count=1000
1000+0 records in
500+0 records out
1048576000 bytes (1.0 GB) copied, 1.74316 s, 602 MB/s

real    0m1.761s
user    0m1.096s
sys     0m0.477s

Pádraig.