EPOX nVIDIA nForce3 250Gb Manuel d'utilisateur télécharger pdf (Page 4)

auxiliary connector (short VSI cable coming out of the PC), instead of the copy of

input which is regularly there. The data output (be it TVG data or duplicated data

from the primary input) of the auxiliary VSI-H connector is unaffected by VSIB

board operation, that is, regardless whether the VSIB board is stopped, waiting, or

running. Please note that the TVG VSI data stream being outputted in this case is

indeed “overclocked” at 50 MHz.

3.2 VSIB Board Operation

The VSIB board has a very simple operating model, one settable 32-bit word of set-

tings and an internal state, one of three possible states: stopped, waiting, running.

Regularly, the board is in the “stopped” state where it resets all its internal logic

to a known state. This is reﬂected in the back panel LED number 1 (and also on-

board D4). At any time the driver software can reset VSIB back to this state by just

writing into the 32-bit settings/mode word.

The board is started by writing another bit combination (the start bit together with

all the settings required) into the 32-bit settings/mode word. The on-board logic

lights up the panel LED number 2 (and also on-board D5) and starts “hunting” for

the next occurrence of VSI 1pps pulse, keeping the logic in this “waiting” state.

The ﬁrst data word being clocked in with the VSI 1pps signal asserted is the ﬁrst

word being captured and this triggers the board into the “running” state and lights

up the panel LED number 3 (and also on-board D6). The board starts pushing in-

coming VSI words into its on-board 4 kB FIFO and from there into the PCI bus us-

ing DMA. The words land into a Linux “bigphysarea” main memory buffer where

the ’vsib.o’ driver ﬁnds them and presents them to the user mode programs via

regular ’read()’ calls. The main memory buffer is organized as a “scatter-gather”

circular buffer and it is allocated when the ’vsib.o’ device driver is loaded with the

argument ’bigbufsize’:

insmod /home/amn/proj/vsib/vsib.o bigbufsize=144000000

The above allocates 144000000 bytes for the ring buffer; generally at 512 Mbps less

than 10 Mbytes is sufﬁcient. The maximum consumption of this is being tracked

with kernel log messages which appear in Linux virtual console number 1 (Alt-F1)

and are also recorded in /var/log/kern.log:

vsib: big secondary ring buffer filled to 3456000 bytes

Should this reach a value very close to 144000000 it very probably indicates that the

disk subsystem is not keeping up with the VSIB PCI DMA. A set of three Maxtor

200GB PATA disks can sustain a little over 700 Mbps at the beginning of disks but

at the inner tracks, during the last 2% of disk capacity the sustained performance

is very close to 512 Mbps. Another typical reason for sudden disk slowness is the

attempt to overwrite large old ﬁles with new ﬁles with the same ﬁle names: delet-

ing large ﬁles “on-the-ﬂy” takes a relatively long time and the disk write process

is left behind the PCI DMA rate.

The panel LED number 4 (and also on-board D7) indicates that the on-board PCI

FIFO has overﬂowed. This could happen if the PCI bus is unable to DMA data

quickly enough from VSIB into main memory. nVidia nForce chipsets are quite

good at this, though, and this problem starts to appear at closer to 800 Mbps.

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EPOX nVIDIA nForce3 250Gb Manuel d'utilisateur Page 4

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