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Serious Performance on the MPX2: the PCI-buses

Leaving this overclocking malarkey to one side . . . 

The major bottleneck in present-day workstations & entry-level servers is not the available CPU-speed [massive] - or the memory bandwidth of DDR [massive again] - it's the PCI-bus, off which run the system's hard disk controllers.

The AMD760 MPX chipset allows at last for an affordable platform with 64-bit/66MHz PCI-slots; in fact it allows for exactly & only two of 'em; & the physical layout of the MPX2 - like that of the Gigabyte competition - gives sufficient space for bulky full-length PCI-cards to be fitted & used.

The importance of 64-bit/66MHz PCI is that this quadruples the theoretical bandwidth available to high-end storage:

 - since current 10K rpm SCSI HD's like the Fujitsu MAN3xx4 series have a peak read-rate of around 55MB/s, & ordinary 7200rpm IDE HD's in the 40'sMB/s; it doesn't take much calculation to see that RAID-arrays of 4 or more of these devices can or should test or exceed the limits of the old 32-bit/33MHz PCI-bus. 

PCI-bus storage-controller performance is much more complex than these crude numbers might suggest; VIA received much unfavourable comment recently for the dire default performance of 32-bit buses off their Southbridges.

One of the keys to resolving these issues turned out to be controlling the PCI-latency [how long a device can hang on to the bus] - PCI-latency issues are also associated with the many problems seen in systems with Creative SB Live audio-accelerators installed. 

Not only did VIA have to release a series of patches to correct this PCI-bus latency issue; but better-quality SMP motherboards using VIA chipsets - like the RioWorks SDVIA - include PCI-latency adjustment within the BIOS: similar adjustment is provided to MPX-chipset motherboards from Asus & Tyan.

We've tested the MPX2 - which by default appears to have 32-clock latency on all its PCI-buses - with 3 different U3W 2-channel 64-bit SCSI RAID-hosts in a 64/66 slot controlling 6-disk RAID0 & RAID5 arrays built from an unchanging hardware configuration of Fujitsu MAN3184 HD's; & with a Promise TX2 IDE s/w RAID-controller with 2-disk & 4-disk RAID0 arrays of Maxtor DX740X HD's in both 64/66 & 32/33 slots: 

The ATTO benchmarks below give a rough initial impression of how well the PCI-bus controllers of the AMD762/768 chipset work - with specific reference to its implementation in the MPX2. 

(Please note these benchmarks are all from a single & quite simple benchmarking utility: we are running IOMeter tests to confirm basic Read & Write transfer rates) 

U3W SCSI RAID0 & RAID5: 64-bit 66MHz:

Compaq SmartArray 5302/64: 6-HD RAID5

SmartArray 5302/64: 6-HD RAID0

U3W SCSI RAID5: 64-bit/33MHz:

IBM ServeRAID-4M: 6HD RAID5

[settings throughout: writeback cache for physical & logical drives; Adaptive readahead cache mode; both channels U160]

U3W SCSI RAID5: 64-bit/33MHz:

ICP-Vortex GDT7523RN: 6HD RAID5

[settings throughout: Synchronous transfer=ON; Synchronous transfer rate =U160; Disk Read Cache = ON; Disk Write cache = ON; Tagged queues = ON]

IDE RAID0: Promise TX2 100 32-bit/66mhz host in 66 & 33MHz slots with 2 & 4 drives

66MHz RAID0

33MHz RAID0

66MHz 4-HD RAID0

66MHz 4-HD RAID0/1

further questions (not conclusions):

IDE: 

The Promise TX2 is a cheap-'n-cheerful s/w RAID-host, dependant on OS-drivers: however, its performance - specifically write-performance - varies markedly between the 66 & 33MHz PCI-buses with no other obvious variable present & when nowhere near theoretical 32-bit bandwidth limits [no other PCI-card was installed during these tests].

Its disastrously poor exploitation of 4 drives in both RAID0 & 0/1 is no doubt partly due to IDE's unhappy combo of one-command-at-a-time & master-slave contention.

We are now testing a 64-bit/33MHz 4-channel hardware IDE RAID-host with 4 Maxtor D740X's [3ware 7410] & will be adding benchmarks to this review.

SCSI: 

The good: Our Compaq SmartArray 5302/64 64-bit/66MHz U3W RAID-host shows acceptable if mysterious performance in the MPX2 [& reportedly in a similar system installed in an MPX chipset MSI K7D]: normally, one would expect RAID5 & RAID0 arrays, using the same hardware, to show contrasting performance-profiles; while those above are near-identical:

Typically; write-performance in RAID5 is markedly slowed in comparison to read-performance [the Xor overhead]; whereas RAID0 - essentially overhead-free - scales fairly predictably from the the individual drive's read & write performance.

The benchmarks above may show a quirk of this enterprise-level host [optimised RAID5/compromised RAID0 performance] or that the MPX2's 64-bit PCI-bus limits performance towards the higher end. 

Two other enterprise-level 2-channel U3W RAID-hosts, both using the Intel i960RN co-processor & LSI 33MHz U3W SCSI-bus silicon [IBM ServeRAID-4M & ICP-Vortex GDT7523RN] showed very poor performance on the MPX2.

The bad: The IBM ServeRAID-4M is a decent-quality 64-bit 2-channel U3W host, very similar to Mylex hosts of similar specification [IBM own Mylex]: 

The ServeRAID-4M's performance when installed into the MPX2 is very poor indeed, despite being optimally set up & using identical hardware to the Compaq 5302.

The ugly: The ICP-Vortex is a supposedly high-performing host; it is much more tweakable than the other hosts tested & we used the optimal settings recommended by ICP-Vortex; again using identical hardware as the Compaq & IBM.

The GDT7523RN's performance when installed in the MPX2 is dreadful - worse than a single 7200rpm IDE HD. 

Latency? I2O?

We have nothing like enough evidence on the MPX2's PCI-bus performance to draw firm conclusions: if we had to give a first opinion based on these simple benchmarks, it would be that it's a disappointment compared to other 64-bit PCI platforms:

However, what we have seen over several week's testing suggests this platform has several issues with commonly used RAID-hosts, both IDE & SCSI:

We suspect that the performance of the Compaq 53xx host on the MPX2 is limited when approaching 200MB/s by PCI-latency issues.

We also suspect the Promise TX2's very different performance between the two PCI-buses of the MPX2 is also a latency issue. 

The dire performance of the two other SCSI RAID-hosts tested on the MPX2 points to a much more profound problem, which seems likely to be common to all motherboards using the AMD MPX chipset: we suspect that there is an issue between this chipset & devices using versions of the I2O protocol - which may be cured to some degree by firmware updates.

We would very much like to see Iwill joining other manufacturers of MPX-chipset motherboards - such as Tyan & Asus - & provide an in-BIOS option to vary the PCI-bus latency [ideally both buses' latencies] to the user's requirements: the MPX2's default setting of 32 clocks is not optimal for a 64-bit/66MHz bus used by higher-end storage controllers - 96 clocks is a not uncommon setting.

We are deeply concerned that the MPX-chipset platform may be at present fundamentally unsuited to use of many Intel i960 co-processor, I2O-architecture SCSI RAID-hosts on the market [this is the most common RAID-host hardware configuration: used by Adaptec & AMI/LSI as well as those mentioned above]. We have seen many reports of similar performance isssues using these & similar hosts on other MPX-chipset motherboards.