Problem - Kevin Tafuro

limits are usually set much higher than they need to be, which ends up allowing any
given application to use up far more resources than it ordinarily should.
Unix provides a mechanism by which an application can self-impose restrictive limits
on the resources that it uses. It’s a good idea for the programmer to lower the limits
to a point where the application can run comfortably, but if something
unexpected happens (such as a memory leak or, more to the point, a denial of service
attack), the limits cause the application to begin failing without bringing down
the rest of the system with it.
Discussion
Operating system resources are difficult for an application to control; the pooling
approach used in threads and sockets is difficult to implement when the application
does not explicitly allocate and destroy its own resources. System resources such as
memory, CPU time, disk space, and open file descriptors are best managed using system
quotas. The programmer can never be sure that system quotas are enabled when
the application is running; therefore, it pays to be defensive and to write code that is
reasonably aware of system resource management.
The most basic advice will be long familiar from lectures on good programming practice:
• Avoid the use of system calls when possible.
• Minimize the number of filesystem reads and writes.
• Steer away from CPU-intensive or “tight” loops.
• Avoid allocating large buffers on the stack.
The ambitious programmer may wish to replace library and operating system
resource management subsystems, by such means as writing a memory allocator that
enforces a maximum memory usage per thread, or writing a scheduler tied to the system
clock which pauses or stops threads and processes with SIGSTOP signals after a
specified period of time. While these are viable solutions and should be considered
for any large-scale project, they greatly increase development time and are likely to
introduce new bugs into the system.
Instead, you may wish to voluntarily submit to the resource limits enforced by system
quotas, thereby in effect “enabling” quotas for the application. This can be done
with the setrlimit( ) function, which allows the resources listed in Table 13-1 to be
limited. Note, however, that not all systems implement all resource limits listed in
this table. Exceeding any of these limits will cause runtime errors such as ENOMEM
when attempting to allocate memory after RLIMIT_DATA has been reached. On BSDderived
systems, two exceptions are RLIMIT_CPU and RLIMIT_FSIZE, which raise the
SIGXCPU and SIGXFSZ signals, respectively.
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