| Fixit7 |
03-11-2016 04:36 PM |
Why PCI Express USB 3.0 card is slower than built-in ports.
Why my PCI Express USB 3.0 card is slower than built-in
ports.
Quote:
So the PCI Express slot is already the gating item.
PCI Express cannot even go at the link rate, due
to limited buffer space on the hub bus buffer. Only
a chipset USB3 port runs at the full rate. The
"add-on" designs are typically passing through
a x1 interface, and then you have to take the
buffer into account.
http://www.plxtech.com/files/pdf/tec...yload_Size.pdf
When a Southbridge USB3 logic block is connected
to the Northbridge, it uses at least the equivalent
of a x4 connection. Even if the connection runs at
50% efficiency, you get a x2 rate, and enough to run
at least a benchmark test at full speed. Whereas,
plugin chips tend to have x1 interfaces, and if the
bus efficiency isn't 100%, then the max USB3 rates
are limited. This is why add-on cards are inferior,
because the company making the chip didn't put an
x2 or x4 interface on it.
And so, a bandwidth report seeing more than is
possible, tells you there is a system file cache
present.
*******
From an administrator account, you could try
echo 3 > /proc/sys/vm/drop_caches
where the possible values are
1 = pagecache
2 = dentries,inodes
3 = both
Doing that, should make no different to
Linux program running speed. But what it
can do, is flush the system file cache.
To be done just before a benchmark step.
To make the benchmark more honest.
Paul
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