Beyond the 32-bit virtual address limit, applications programmers can still handle large
problem sizes. Usually, applications that exceed the 32-bit virtual address limit split the
application data set between primary memory and secondary memory, for example, onto a
disk. Unfortunately, transferring data to and from a disk drive takes a longer time, in orders of
magnitude, than memory-to-memory transfers.
Today, many servers can handle more than 4 Gigabytes of physical memory. High-end desktop
machines are following the same trend, but no single 32-bit program can directly address more
than 4 Gigabytes at a time. However, a 64-bit application can use the 64-bit virtual address space
capability to allow up to 18 Exabytes (1 Exabyte is approximately 10
18
bytes) to be directly
addressed. Thus, larger problems can be handled directly in primary memory. If the application
is multithreaded and scalable, then more processors can be added to the system to speed up the
application even further. Such applications become limited only by the amount of physical
memory in the machine.
It might seem obvious, but for a broad class of applications, the ability to handle larger problems
directly in primary memory is the major performance benet of 64-bit machines.
■
A greater proportion of a database can live in primary memory.
■
Larger CAD/CAE models and simulations can t in primary memory.
■
Larger scientic computing problems can t in primary memory.
■
Web caches can hold more in memory, reducing latency.
FIGURE 1–1 Typical Performance and Problem Size Curve
Relative
Performance
Log Problem Size
D$
E$
hw PA limit
32-bit VA limit
Getting Past the 4 Gigabyte Barrier
Solaris 64-bit Developer's Guide • January 200512
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