Increasing MD5 GPU Utilization?

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Hi

Was hoping for some pointers with this amazing bit of software.

Running Windows 7 64 Bit ,Catalyst 14.4, oclHashcat-1.20
2x7970 GPUs

Benchmark for MD5 below;
Hashtype: MD5
Workload: 1024 loops, 256 accel

Speed.GPU.#1.: 9478.4 MH/s
Speed.GPU.#2.: 8697.6 MH/s
Speed.GPU.#*.: 18176.0 MH/s

I have a situation were the plain text for part of a string is known, with some unknown details in the middle. This isn't the real string that the hash is built from, but it performs the same way. Let's call it "Some known text?1?1?1?1?1?1?1?1more known text".

Have used the following to attempt to solve it. Have tried various lengths.

oclHashcat64.exe -a 3 -m 0 -1 ?u?l?d -n 256 -u 1024 6xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx5 "Some known text?1?1?1?1?1?1?1?1more known text"

This command delivered the throughput below, which I thought was good until I saw the MD5 benchmarks for my rig. The GPU util rates are not that high.

Session.Name...: oclHashcat
Status.........: Running
Input.Mode.....: Mask (Some known text?1?1?1?1?1?1?1?1more known text) [38]
Hash.Target....: 6xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx5
Hash.Type......: MD5
Time.Started...: Mon May 12 13:08:49 2014 (15 secs)
Time.Estimated.: Thu May 22 16:17:09 2014 (10 days, 3 hours)
Speed.GPU.#1...: 137.2 MH/s
Speed.GPU.#2...: 114.3 MH/s
Speed.GPU.#*...: 251.5 MH/s
Recovered......: 0/1 (0.00%) Digests, 0/1 (0.00%) Salts
Progress.......: 3902799872/218340105584896 (0.00%)
Skipped........: 0/3902799872 (0.00%)
Rejected.......: 0/3902799872 (0.00%)
HWMon.GPU.#1...: 80% Util, 69c Temp, 87% Fan
HWMon.GPU.#2...: 83% Util, 61c Temp, 73% Fan

Doing some testing to work out how to increase the throughput to a level comparable to those of the benchmark, I was curious as to the effect of removing the text off the front of the test string to observe the effect on GPU utilization - though it wouldn't solve the hash. I shortened the test string to

"?1?1?1?1?1?1?1?1more known text"


oclHashcat64.exe -a 3 -m 0 -1 ?u?l?d -n 256 -u 1024 6xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx5 "?1?1?1?1?1?1?1?1more known text"

The throughput went up massively and the GPU utilization climbed significantly.

Session.Name...: oclHashcat
Status.........: Running
Input.Mode.....: Mask (?1?1?1?1?1?1?1?1more known text) [23]
Hash.Target....: 6xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx5
Hash.Type......: MD5
Time.Started...: Mon May 12 13:10:08 2014 (18 secs)
Time.Estimated.: Mon May 12 16:31:33 2014 (3 hours, 21 mins)
Speed.GPU.#1...: 9545.0 MH/s
Speed.GPU.#2...: 8679.0 MH/s
Speed.GPU.#*...: 18224.0 MH/s
Recovered......: 0/1 (0.00%) Digests, 0/1 (0.00%) Salts
Progress.......: 342246817792/218340105584896 (0.16%)
Skipped........: 0/342246817792 (0.00%)
Rejected.......: 0/342246817792 (0.00%)
HWMon.GPU.#1...: 98% Util, 75c Temp, 94% Fan
HWMon.GPU.#2...: 98% Util, 66c Temp, 78% Fan

I am trying to understand why this massive change in throughput occurred, and in general, how to speed the MD5 search of a key space similar to the one below.

"Some known text?1?1?1?1?1?1?1?1more known text"

Any suggestions would be appreciated.