PCI capping? or am I missing something?

Liamalxd · 3 Sep 2012 at 12:35

I was pondering the other day about this and wondered about the PCI slots we all seem to have in modern-ish computer systems.

Say your PCI slot is the standard speed, thats 133mb/s. (according to wiki haha)

So how in the world does a gigabit NIC work properly in it?
Or does it just show as 1gb/s and run at 133mb/s?

Would love to hear your thoughts...

bemaniac · 3 Sep 2012 at 12:39

Isn't 1gb ethernet 100MB/S

Liamalxd · 3 Sep 2012 at 12:41

bemaniac said:
Isn't 1gb ethernet 100MB/S

good point ... hmm, maybe i'm a victim of the megabits vs megabytes argument

ubersonic · 3 Sep 2012 at 13:00

Old PCI slots are 133MB/s (assuming the normal 32bit ones in desktop boards) and gigabit is 125MB/s (bytes / 8 = bits).

But if you do see a "gigabit" NIC for a standard 133MB/s slot then no it wont operate at full speed even though gigabit is 125MB/s, because the 133MB limit is for the whole PCI bus not just that slot.

Liamalxd · 13 Sep 2012 at 12:06

ubersonic said:
Old PCI slots are 133MB/s (assuming the normal 32bit ones in desktop boards) and gigabit is 125MB/s (bytes / 8 = bits).

But if you do see a "gigabit" NIC for a standard 133MB/s slot then no it wont operate at full speed even though gigabit is 125MB/s, because the 133MB limit is for the whole PCI bus not just that slot.

After thinking about this for a while, figured out that the gigabit NIC is in bits not bytes so the 125mb/s speed you were talking about divide it by 8, (since theres 8 bits in a byte) that gives us 15.625 MegaBYTES per second. which is the usual speed you receive when doing a gigabit to gigabit large file transfer over a network.

so the 133mb/s for the PCI bus could technically support about 8 gigabit cards before capping out...

simples

bremen1874 · 13 Sep 2012 at 12:15

Liamalxd said:
After thinking about this for a while, figured out that the gigabit NIC is in bits not bytes so the 125mb/s speed you were talking about divide it by 8, (since theres 8 bits in a byte) that gives us 15.625 MegaBYTES per second. which is the usual speed you receive when doing a gigabit to gigabit large file transfer over a network.

so the 133mb/s for the PCI bus could technically support about 8 gigabit cards before capping out...

simples

And wrong. You divide Gigabit by 8 to get the 125MB/s figure, you don't want to divide it by 8 again.

If it was true all that messing around with PCI-X network cards to get sufficient bandwidth would be a bit of a waste of time.

Ch3m1c4L · 13 Sep 2012 at 16:18

Liamalxd said:
After thinking about this for a while, figured out that the gigabit NIC is in bits not bytes so the 125mb/s speed you were talking about divide it by 8, (since theres 8 bits in a byte) that gives us 15.625 MegaBYTES per second. which is the usual speed you receive when doing a gigabit to gigabit large file transfer over a network.

so the 133mb/s for the PCI bus could technically support about 8 gigabit cards before capping out...

simples

Also, what you on about only getting 15MB/sec on a Gbit network? I get 80-90, sometimes 99. Speeds were observed on transfers, after around an hour, on sizes of 300GB to 1.2TB (steam directory transfer to my second machine).