Re: packet drop with intel gigabit / marwell gigabit

--- "Jin Guojun [VFFS]" <g_jin@xxxxxxx> wrote:
Arne Woerner wrote:
What did we show by this <<dd if=/dev/zero ...>> test? I
thoughtthat
would prove the memory bandwidth is about 8Gbit/sec(1GByte/sec;
2 * <dd's bytes/sec >>number>/2^30).

It depends on how you use /dev/zero.
dd of=/dev/null if=/dev/zero bs=4k count=100k
tests cache speed
% dd of=/dev/null if=/dev/zero bs=4k count=100k
102400+0 records in
102400+0 records out
419430400 bytes transferred in 0.204511 secs (2050894814
bytes/sec)
about 32Gbit/sec?

dd of=/dev/null if=/dev/zero bs=4m count=100
tests memory bandwidth if your cache is less than 2 MB

% dd of=/dev/null if=/dev/zero bs=4m count=100
100+0 records in
100+0 records out
419430400 bytes transferred in 2.587341 secs (162108677 bytes/sec)
about 2.4Gbit/sec?

I had an mpeg encoder in the background, when i did those
benchmarks... :-)

Now you may give me the real memory bandwidth on your
system :-)
I would expect something around 500.

Hmm... 500Mbit/sec? even if i divide 2.4Gbit by 4, i still get
600Mbit/sec on a quite busy (50%) system...

Oh... The docu TV series episode is over now and I re-ran the
tests:
% dd of=/dev/null if=/dev/zero bs=4m count=100
100+0 records in
100+0 records out
419430400 bytes transferred in 1.513610 secs (277106012 bytes/sec)
% dd of=/dev/null if=/dev/zero bs=4k count=500k
512000+0 records in
512000+0 records out
2097152000 bytes transferred in 0.945430 secs (2218199591
bytes/sec)

Notice that your memory copy speed will be one half of it.

Why "half"? dd causes two copies but counts each byte just once...

Maybe "dd" in combination with /dev/zero is not the right way to
measure memory bandwidth?

-Arne


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