Why is crossfire slower than a single card?

[rageltman]

Good afternoon. I just purchased a pair of ATI 6970M cards for my 'laptop' (clevo x7200). And i'm seeing some very strange behavior.

I run ubuntu 11.04, installed the newest ATI drivers from their site (11.11), configured crossfire, had to use the --acpi-services=off flag just to get GDM to load, and viola, it runs.

oclhashcat+ though, is a different story. It sees both of the GPUs, but runs slower with both than with one:

SINGLE RUN:

Hashes: 233
Unique digests: 233
Bitmaps: 11 bits, 2048 entries, 0x000007ff mask, 8192 bytes
Rules: 3141
GPU-Loops: 128
GPU-Accel: 40
Password lengths range: 1 - 15
Platform: AMD compatible platform found
Watchdog: Temperature limit set to 90c
Device #1: Barts, 2048MB, 0Mhz, 12MCU
Device #2: skipped by user
Device #1: Allocating 72MB host-memory
Device #1: Kernel ./kernels/4098/m1000_a0.Barts.64.kernel (1108212 bytes)

Status.......: Running
Input.Mode...: File (/tmp/51083936)
Hash.Type....: NTLM
Time.Running.: 6 secs
Time.Left....: 3 mins, 7 secs
Time.Util....: 6815.8ms/155.8ms Real/CPU, 2.3% idle
Speed........: 827.4M c/s Real, 894.1M c/s GPU
Recovered....: 0/233 Digests, 0/1 Salts
Progress.....: 5639461002/160454642976 (3.51%)
Rejected.....: 6282/5639461002 (0.00%)
HW.Monitor.#1: 96% GPU, 66c Temp


Hashes: 240
Unique digests: 240
Bitmaps: 11 bits, 2048 entries, 0x000007ff mask, 8192 bytes
Rules: 3141
GPU-Loops: 128
GPU-Accel: 40
Password lengths range: 1 - 15
Platform: AMD compatible platform found
Watchdog: Temperature limit set to 90c
Device #1: Barts, 2048MB, 0Mhz, 12MCU
Device #2: Barts, 2048MB, 0Mhz, 12MCU
Device #1: Allocating 72MB host-memory
Device #1: Kernel ./kernels/4098/m1000_a0.Barts.64.kernel (1108212 bytes)
Device #2: Allocating 72MB host-memory
Device #2: Kernel ./kernels/4098/m1000_a0.Barts.64.kernel (1108212 bytes)

Scanned dictionary /tmp/51083936: 495738527 bytes, 160454642976 words, starting attack...

[s]tatus [p]ause [r]esume [q]uit => s
Status.......: Running
Input.Mode...: File (/tmp/51083936)
Hash.Type....: NTLM
Time.Running.: 56 secs
Time.Left....: 2 mins, 23 secs
Time.Util....: 56691.8ms/1024.3ms Real/CPU, 1.8% idle
Speed........: 802.5M c/s Real, 809.5M c/s GPU
Recovered....: 7/240 Digests, 0/1 Salts
Progress.....: 45495589002/160454642976 (28.35%)
Rejected.....: 6282/45495589002 (0.00%)
HW.Monitor.#1: 49% GPU, 71c Temp
HW.Monitor.#2: 49% GPU, 71c Temp


Something is very wrong here. Config:
aticonfig --lscs
Candidate Combination:
Master: 4:0:0
Slave: 3:0:0
CrossFire is enabled on current device
CrossFire Diagnostics:
CrossFire can work with P2P mapping through GART
Dongle Capabilities: support PASSTHROUGH |INTERLINK_SW_AFR | INTERLINK_AUTO_AFR | INTERLINK_BLACKING | INTERLINK_SUPERAA

aticonfig --lscc

Master adapter: 0. 04:00.0 AMD Radeon HD 6900M Series
Candidates: 1. 03:00.0 AMD Radeon HD 6900M Series
aticonfig --lsch

CrossFire chain for adapter 0, status: enabled
0. 04:00.0 AMD Radeon HD 6900M Series
1. 03:00.0 AMD Radeon HD 6900M Series