IBM Power System POWER7 Series, Optimization And Tuning Manual

Page: 67 / 224

Chapter 2. The POWER7 processor
51
To set the operating system default prefetch depth on the system temporarily (that is, for the
current session) or permanently (that is, after each reboot), run the following command:
dscrctl [-n] [-b] -s
<dscr_value>
The dscr_value is treated as a decimal number unless it starts with 0x, in which case it is
treated as hexadecimal.
To cancel a permanent setting of the operating system default prefetch depth at boot time, run
the following command:
dscrctl -c
Applications that have predictable data access patterns, such as numerical applications that
process arrays of data in a sequential manner, benefit from aggressive data prefetching.
These applications must run with the default operating system prefetch depth, or whichever
settings are empirically found to be the most beneficial.
Applications that have considerably unpredictable data access patterns, such as some
transactional applications, can be negatively affected by aggressive data prefetching. The
data that is prefetched is unlikely to be needed, and the prefetching uses system bandwidth
and might displace useful data from the caches. Some WebSphere Application Server and
DB2 workloads have this characteristic. Performance can be improved by disabling hardware
prefetching in these cases by running the following command:
dscrctl -n -s 1
This system (partition) wide disabling is only appropriate if it is expected to benefit all of the
applications that are running in the partition. However, the same effect can be achieved on an
application-specific basis by using the programming API.
For more information about this topic, see 2.4, “Related publications” on page 51.
2.4 Related publications
The publications that are listed in this section are considered suitable for a more detailed
discussion of the topics that are covered in this chapter:
򐂰 AIX dscr_ctl API sample code, found at:
https://www.power.org/documentation/performance-guide-for-hpc-applications-on-i
bm-power-755-system/
(registration required)
򐂰
AIX Version 7.1 Release Notes, found at:
http://publib.boulder.ibm.com/infocenter/aix/v7r1/index.jsp?topic=/com.ibm.aix.
ntl/RELNOTES/GI11-9815-00.htm
Refer to the section, The
dscrctl
command.
򐂰 Application configuration for large pages, found at:
http://publib.boulder.ibm.com/infocenter/aix/v7r1/index.jsp?topic=/com.ibm.aix.
prftungd/doc/prftungd/config_apps_large_pages.htm
򐂰 False Sharing, found at:
http://msdn.microsoft.com/en-us/magazine/cc872851.aspx
򐂰 lwsync instruction, found at:
http://pic.dhe.ibm.com/infocenter/aix/v7r1/topic/com.ibm.aix.aixassem/doc/alang
ref/idalangref_sync_dcs_instrs.htm