Linux application crashes, no core dump while “ulimit -c unlimited” and “/proc/sys/kernel/core_pattern” are set

Essentially /proc and /sys are the same.

sysfs was added in kernel 2.5 or 2.6 due to clutter in procfs

The procfs was only meant to hold process information. eventually everything started getting mixed into proc and it created a twisty maze with device data stuck in different spots all over the place. Meanwhile, sysfs was implemented with the objective of segmenting device data from procfs.

Specifically, /sys maintains more detailed (position of nodes actually represents the device hierarchy by subsystem) device process information. For each object in the driver model, a directory is created. The device file structure being:

/sys/devices  - devices by physical layout
/sys/bus      - symbolic links to devices
/sys/block    - devices by block
/sys/class    - devices by class

On your local system you might find man page at man sysfs and information about modifying kernel parameters in /proc/sys with man sysctl

If you want even more information about /sys, you might want to view the Linux sysfs filesystem whitepaper on kernel.org [pdf] and Documentation/sysfs.txt in the kernel source.

file-max is the maximum File Descriptors (FD) enforced on a kernel level, which cannot be surpassed by all processes without increasing. The ulimit is enforced on a process level, which can be less than the file-max.

There is no performance impact risk by increasing file-max. Modern distributions have the maximum FD set pretty high, whereas in the past it required kernel recompilation and modification to increase past 1024. I wouldn't increase system-wide unless you have a technical need.

The per-process configuration often needs tuned for serving a particular daemon be it either a database or a Web server. If you remove the limit entirely, that daemon could potentially exhaust all available system resources; meaning you would be unable to fix the problem except by pressing the reset button or power cycling. Of course, either of those is likely to result in corruption of any open files.