Why limit file descriptors

A good value is Setting the max open file descriptors and max user processes is done via the following call:. See increase-open-files-limit for more details. Introduction: Challenges like number of open files in any of the production environment has become common now a day. Since many applications which are Java based and Apache based, are getting installed and configured, which may lead to too many open files, file descriptors etc.

If this exceeds the default limit that is set, then one may face access control problems and file opening challenges. Many production environments come to standstill kind of situations because of this. This command is equipped with many options and with this combination one can set number of open files.

Following are step-by-step commands with examples explained in detail. This clearly indicates that individual Linux operating systems have different number of open files. Why do operating systems put limits on the number of open file descriptors a process and the global system can have? My current server implementation uses zeromq, and opens a subscriber socket for each connected websocket client.

Obviously that single process is only going to be able to handle clients to the limit of the fds. When I research the topic I find lots of info on how to raise system limits to levels as high as 64k fds but it never mentions how it affects system performance and why it is 1k and lower to start with?

My current approach is to try and dispatch messaging to all clients using a coroutine in its own loop, and a map of all clients and their subscription channels.

But I would just love to hear a solid answer about file descriptor limitations and how they affect applications that try to use them on a per client level with persistent connections? It may be because a file descriptor value is an index into a file descriptor table. Therefore, the number of possible file descriptors would determine the size of the table. Average users would not want half of their ram being used up by a file descriptor table that can handle millions of file descriptors that they will never need.

For performance purposes, the open file table needs to be statically allocated, so its size needs to be fixed. File descriptors are just offsets into this table, so all the entries need to be contiguous. You can resize the table, but this requires halting all threads in the process and allocating a new block of memory for the file table, then copying all entries from the old table to the new one.

It's not something you want to do dynamically, especially when the reason you're doing it is because the old table is full! There are certain operations which slow down when you have lots of potential file descriptors. This isn't a particularly good reason, but it is a reason. Another reason is simply to keep a buggy program i. Since this is the unix api for launching a process, it has to be done anytime a new process is created, including executing each and every non built-in command invoked within shell scripts.

It doesn't scale well for thousands of files as these descriptor sets need to be setup and cleared each time around the runloop. I haven't used aio in anger and it certainly wouldn't have the performance and semantics offered by native approaches in the way they can aggregate multiple events for higher performance.

The upshot is that increasing the number of allowed open files across the system adds both time and space overhead for every process in the system even if they can't make use of those descriptors based on the api they are using. There are also aggregate system limits as well for the number of open files allowed. This older but interesting tuning summary for kk simultaneous connections using nginx on FreeBSD provides some insight into the overheads for maintaining open connections and another one covering a wider range of systems but "only" seeing 10K connections as the Mt Everest.

Probably the best reference for unix system programing is W. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more.