Contents | Previous | Next

QDIO Enhanced Buffer State Management

Introduction:  

The Queued Direct I/O (QDIO) Enhanced Buffer State Management (QEBSM) facility provides an optimized mechanism for transferring data via QDIO (including FCP, which uses QDIO) to and from virtual machines. Prior to this new facility, z/VM had to mediate between the virtual machine and the OSA Express or FCP adapter during QDIO data transfers. With QEBSM, z/VM is not involved with a typical QDIO data transfer when the guest operating system or device driver supports the facility.

Starting with z/VM 5.2.0 and the z990/z890 with QEBSM Enablement applied (refer to Performance Related APARs for a list of required maintenance), a program running in a virtual machine has the option of using QEBSM when performing QDIO operations. By using QEBSM, the processor millicode performs the shadow-queue processing typically performed by z/VM for a QDIO operation. This eliminates the z/VM and hardware overhead associated with SIE entry and exit for every QDIO data transfer. The shadow-queue processing still requires processor time, but much less than required when done by the software. The net effect is a small increase in virtual CPU time coupled with a much larger decrease in CP CPU time.

This section summarizes measurement results comparing Linux communicating over a QDIO connection under z/VM 5.1.0 with measurement results under z/VM 5.2.0 with QEBSM active.

Methodology:  

The Application Workload Modeler (AWM) was used to drive the workload for OSA and HiperSockets. (Refer to AWM Workload for more information.) A complete set of runs was done for RR and STR workloads. IOzone was used to drive the workload for native SCSI (FCP) devices. Refer to Linux IOzone Workload for details.

The measurements were done on a 2084-324 with two dedicated processors in each of the two LPARs used. Running under z/VM, an internal Linux driver at level 2.6.14-16 that supports QEBSM was used.

Two LPARs were used for the OSA and HiperSockets measurements. The AWM client ran in one LPAR and the AWM server ran in the other LPAR. Each LPAR had 2GB of main storage and 2GB of expanded storage. CP Monitor data were captured for one LPAR (client side) during the measurement and were reduced using Performance Toolkit for VM (Perfkit).

One LPAR was used for the FCP measurements. CP Monitor data and hardware instrumentation data were captured.

Summary of Results:  

The direct effect of QEBSM is to decrease CPU time. This, in turn, increases throughput in cases where it had been limited by CPU usage. This effect is demonstrated in this table for all three cases.

Detailed Results:  

The following tables compare the measurements for OSA, HiperSockets and FCP. The %diff numbers shown are the percent increase (or decrease) between the measurement on 5.1.0 and 5.2.0.

Table 1. OSA RR

Table 2. OSA - STR

Table 3. HiperSockets RR

Table 4. HiperSockets - STR

The following table shows the results for FCP. Values are provided for total microseconds (µsec) per transaction, CP (µsec) per transaction, and virtual (µsec) per transaction.

Table 5. FCP

Contents | Previous | Next