以前玩K7時,華碩的MB大多將PCI Latency Timer設成32,最近換了K8,發現華碩K8 MB將PCI Latency Timer預設成64了,於是上網查了一下PCI Latency Timer的說明,找到如下:

PCI Latency Timer

Common Options : 0 - 255

Quick Review

This BIOS feature controls how long a PCI device can hold the PCI bus before another takes over. The longer the latency, the longer the PCI device can retain control of the bus before handing it over to another PCI device.

Normally, the PCI Latency Timer is set to 32 cycles. This means the active PCI device has to complete its transactions within 32 clock cycles or hand it over to the next PCI device.

For better PCI performance, a longer latency should be used. Try increasing it to 64 cycles or even 128 cycles. The optimal value for every system is different. You should benchmark your PCI cards' performance after each change to determine the optimal PCI latency time for your system.

Please note that a longer PCI latency isn't necessarily better. A long latency can also reduce performance as the other PCI devices queuing up may be stalled for too long. This is especially true with systems with many PCI devices or PCI devices that continuously write short bursts of data to the PCI bus. Such systems would work better with shorter PCI latencies as they allow rapid access to the PCI bus.

In addition, some time-critical PCI devices may not agree with a long latency. Such devices require priority access to the PCI bus which may not be possible if the PCI bus is held up by another device for a long period. In such cases, it is recommended that you keep to the default PCI latency of 32 cycles.
來源出處
簡單的說,PCI Latency Timer指的是PCI裝置能夠佔用PCI通道的時間,設成32,就是能夠佔用PCI通道32個週期,設成64,就是佔用PCI通道64個週期,文章中說到,將PCI Latency Timer設長一點,優點是能夠使PCI裝置充分利用到PCI通道,缺點是,其他PCI裝置要排隊使用PCI通道的等候時間變長了.究竟是設32好,還是64好,用測試軟體去測試PCI裝置來決定比較準

於是我想來想去,想到HD也是走PCI bus的,於是我就用PCmark 04中的HDD測試來跑,分別設32跟64各跑二次,選出哪一個設定值分數較高,結果是
PC1: 華碩A8V Deluxe(南橋8237)+Adaptec 29320A-R+Seagate 15.3K 36GB(第一顆)+Seagate 10.6K 73GB(第二顆)=設成64分數較高,最高為5437分
PC2: 華碩P4C800-E Deluxe(南橋ICH5-R)+WD 800JB(單碟80GB,Master)+WD 800JB(非單碟80GB,Slave)=設成64分數較高,最高為4012分
PC3:華碩K8V Deluxe(南橋8237)+Adaptec 29160+Fujitsu MAP3367NP(第一顆)+Hitachi Ultrastar 146Z10(第二顆)=設成64分數較高,約4985分
PC4 華碩K8V Deluxe(南橋8237)+Hitachi 7K250 80GB S-ATA(系統碟)+Maxtor 金鑽八代 IDE 6E040L0(做第二顆)=設成32分數較高,約4329分

這是我搞了一下午的結果,設成32或是64,在PCmark 04中的HDD test上分數真的有差,而且不是只差幾分而已,可以差到五,六十分,不知用HD來test PCI Latency Timer對不對,或是有沒有意義,小弟才疏學淺,只能想到這樣,請大家不吝指教,感謝了