PCI-E Max Read Request Size – The Tech ARP BIOS Guide

PCI-E Max Read Request Size

Common Options : Automatic, Manual – User Defined

 

Quick Review of PCI-E Max Read Request Size

This BIOS feature can be used to ensure a fairer allocation of PCI Express bandwidth. It determines the largest read request any PCI Express device can generate. Reducing the maximum read request size reduces the hogging effect of any device with large reads.

PCI-E Max Read Request Size - The Tech ARP BIOS Guide

When set to Automatic, the BIOS will automatically select a maximum read request size for PCI Express devices. Usually, this would be a manufacturer-preset value that’s designed with maximum “fairness“, rather than performance in mind.

When set to Manual – User Defined, you will be allowed to enter a numeric value (in bytes). Although it appears as though you can enter any value, you must only enter one of these values :

128 – This sets the maximum read request size to 128 bytes. All PCI Express devices will only be allowed to generate read requests of up to 128 bytes in size.

256 – This sets the maximum read request size to 256 bytes. All PCI Express devices will only be allowed to generate read requests of up to 256 bytes in size.

512 – This sets the maximum read request size to 512 bytes. All PCI Express devices will only be allowed to generate read requests of up to 512 bytes in size.

1024 – This sets the maximum read request size to 1024 bytes. All PCI Express devices will only be allowed to generate read requests of up to 1024 bytes in size.

2048 – This sets the maximum read request size to 2048 bytes. All PCI Express devices will only be allowed to generate read requests of up to 2048 bytes in size.

4096 – This sets the maximum read request size to 4096 bytes. This is the largest read request size currently supported by the PCI Express protocol. All PCI Express devices will be allowed to generate read requests of up to 4096 bytes in size.

It is recommended that you set this BIOS feature to 4096, as it maximizes performance by allowing all PCI Express devices to generate as large a read request as they require. However, this will be at the expense of devices that generate smaller read requests.

Even so, this is generally not a problem unless they require a certain degree of quality of service. For example, you may experience glitches with the audio output (e.g. stuttering) of a PCI Express sound card when its reads are delayed by a bandwidth-hogging graphics card.

If such problems arise, reduce the maximum read request size. This reduces the amount of bandwidth any PCI Express device can hog at the expense of the other devices.

 

Details of PCI-E Max Read Request Size

Arbitration for PCI Express bandwidth is based on the number of requests from each device. However, the size of each request is not taken into account. As such, if some devices request much larger data reads than others, the PCI Express bandwidth will be unevenly allocated between those devices.

This can cause problems for applications that have specific quality of service requirements. These application may not have timely access to the requested data simply because another PCI Express device is hogging the bandwidth by requesting for very large data reads.

This BIOS feature can be used to correct that and ensure a fairer allocation of PCI Express bandwidth. It determines the largest read request any PCI Express device can generate. Reducing the maximum read request size reduces the hogging effect of any device with large reads.

However, doing so reduces the performance of devices that generate large reads. Instead of generating large but fewer reads, they will have to generate smaller reads but in greater numbers. Because arbitration is done according to the number of requests, they will have to wait longer for the data requested.

When set to Automatic, the BIOS will automatically select a maximum read request size for PCI Express devices. Usually, this would be a manufacturer-preset value that’s designed with maximum “fairness“, rather than performance in mind.

When set to Manual – User Defined, you will be allowed to enter a numeric value (in bytes). Although it appears as though you can enter any value, you must only enter one of these values :

128 – This sets the maximum read request size to 128 bytes. All PCI Express devices will only be allowed to generate read requests of up to 128 bytes in size.

256 – This sets the maximum read request size to 256 bytes. All PCI Express devices will only be allowed to generate read requests of up to 256 bytes in size.

512 – This sets the maximum read request size to 512 bytes. All PCI Express devices will only be allowed to generate read requests of up to 512 bytes in size.

1024 – This sets the maximum read request size to 1024 bytes. All PCI Express devices will only be allowed to generate read requests of up to 1024 bytes in size.

2048 – This sets the maximum read request size to 2048 bytes. All PCI Express devices will only be allowed to generate read requests of up to 2048 bytes in size.

4096 – This sets the maximum read request size to 4096 bytes. This is the largest read request size currently supported by the PCI Express protocol. All PCI Express devices will be allowed to generate read requests of up to 4096 bytes in size.

It is recommended that you set this BIOS feature to 4096, as it maximizes performance by allowing all PCI Express devices to generate as large a read request as they require. However, this will be at the expense of devices that generate smaller read requests.

Even so, this is generally not a problem unless they require a certain degree of quality of service. For example, you may experience glitches with the audio output (e.g. stuttering) of a PCI Express sound card when its reads are delayed by a bandwidth-hogging graphics card.

If such problems arise, reduce the maximum read request size. This reduces the amount of bandwidth any PCI Express device can hog at the expense of the other devices.

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