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For a Linux server monitoring a very large set of mostly idle connections, epoll is generally the best fit. Unlike select and poll, which receive the watched descriptors again on each wait, epoll keeps registrations in a kernel-managed instance and returns events for descriptors that are ready. That design is a strong reason to choose it for large descriptor sets—not a guarantee that any particular machine can support 100,000 live connections.
What changes when you wait for I/O?
These APIs report readiness: whether an operation such as reading or writing can proceed without blocking. They do not perform the application’s network protocol work, and readiness does not mean that a full message or write will complete in one operation.
select: pass descriptor sets on every wait
The application supplies read, write, and exception descriptor sets, along with an nfds boundary. Those sets are value-result arguments, so the caller must rebuild or restore them between calls. The glibc fd_set interface has a fixed FD_SETSIZE of 1,024: descriptor numbers must be below 1,024 to be monitored through that interface. This is a limit of the glibc interface, not a universal 1,024-descriptor limit imposed by the Linux kernel’s select system call. The Linux man-pages project says modern applications should use poll or epoll instead of select to avoid this limitation.
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The application supplies an array of pollfd records describing the descriptors and requested events. Unlike glibc’s fd_set interface, poll does not have that fixed 1,024-descriptor-number limit. But, like select, the application supplies the watched set on each wait. See the Linux poll(2) manual.
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epoll: register once, retrieve ready events
The application creates an epoll instance, adds or changes watched descriptors with epoll_ctl, and collects ready events with epoll_wait. The kernel maintains an interest list of registered descriptors and a ready list populated as watched descriptors become active. The application does not resupply the full interest list on every wait. The Linux epoll(7) manual describes the API as one that “scales well to large numbers of watched file descriptors.”
How the three APIs compare
| API | What the application supplies when waiting | Watched-set behavior | Key trade-off |
|---|---|---|---|
select |
Read, write, and exception descriptor sets plus nfds |
Sets are passed again and must be rebuilt or restored after calls | Portable, but the glibc fd_set interface cannot monitor descriptor numbers 1,024 and higher |
poll |
An array of pollfd records |
Array is passed again on each wait | Portable and avoids the fixed fd_set size limit, but still resubmits the watched set |
epoll |
An epoll instance and a maximum number of returned events | Registrations persist in the kernel until changed or removed | Linux-specific; suited to large sets where many descriptors are idle |
In the documented model, select and poll involve passing the watched collection to the kernel and checking it for readiness on each wait. With epoll, the registered set persists and the application retrieves ready events. This explains why epoll often suits a large set with relatively few active connections. It is a design-level comparison, not a universal complexity guarantee or a benchmark: actual performance depends on workload, implementation, and system configuration. Michael Kerrisk’s epoll teaching slides describe these recurring operations.
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Does epoll guarantee 100,000 concurrent connections?
No. The API’s scaling model does not establish a machine’s connection capacity. Supporting 100,000 live connections depends on host and process limits, memory, connection state, traffic, application work, and implementation. The cited documentation does not provide a controlled benchmark for exactly 100,000 connections, so there is no supported universal throughput, latency, CPU, memory, or speedup figure to quote.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThere is a documented memory estimate specifically for epoll registrations: the Linux man-pages project gives approximately 90 bytes per registered descriptor on a 32-bit kernel and 160 bytes on a 64-bit kernel. These are rough per-watch estimates, not the total memory cost of a connection. They do not include all resources needed by sockets, buffers, application state, or the rest of the server.
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The same manual documents the default-setting rule for fs.epoll.max_user_watches as one quarter of available low memory divided by the registration cost. That is a rule for the default setting, not a promise that a particular host has a given number of available watches. Check the running host’s configuration and resource limits before treating any figure as operational capacity.
Which API should you choose?
- Choose epoll for a Linux-specific server that monitors a very large number of descriptors, especially when most are idle and only a subset becomes ready at a time.
- Choose poll when you need a portable interface without
select’s glibcfd_setceiling, and the watched set is manageable for your workload. - Use select mainly when compatibility with existing code or a small descriptor set matters more than the fixed glibc interface limit.
- Prefer level-triggered epoll initially if you want epoll’s persistent registration model without the additional discipline required by edge-triggered operation.
select and poll are longstanding portable interfaces; epoll is Linux-specific. If your program must support other operating systems, choose an abstraction or platform-specific implementation strategy that accounts for that difference.
Level-triggered and edge-triggered epoll
Epoll is level-triggered by default, and it can also use edge-triggered delivery with EPOLLET. In level-triggered mode, its readiness semantics are the same as poll, according to the epoll manual. If a descriptor remains ready, it can continue to be reported; this is generally the simpler mode to use correctly.
Edge-triggered mode requires draining the descriptor
With EPOLLET, use nonblocking descriptors and keep reading or writing until the operation returns EAGAIN before waiting for another event. If the application reads only part of the available data and waits for a new edge, it can block even though unread data remains. Edge-triggered operation can reduce notification work in suitable event loops, but it requires careful handling of partial reads and writes.
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One-shot events need rearming
EPOLLONESHOT disables a descriptor after an event is delivered. To receive events for it again, rearm it with epoll_ctl using EPOLL_CTL_MOD. This behavior can be useful when coordinating event handling, but it adds an explicit rearming step.
Handling batches of ready events
epoll_wait returns up to the maximum number of events requested by the caller. If more descriptors are ready than fit in that batch, successive calls rotate through the ready set to help avoid starvation. Set the batch size to match how the application processes work rather than assuming one call returns every ready descriptor. Use the event’s user-data field to associate each result with the relevant descriptor or connection state. See the Linux epoll_wait(2) manual.
Check limits on the server you will deploy
For a high-connection-count service, inspect the target host rather than relying on a generic capacity number. In particular, verify the process and host resource limits, available memory, socket and connection conditions, and the configured epoll watch limit. The max_user_watches setting is host- and kernel-dependent; its documented default-setting rule does not tell you the current effective value on your server. Also account for the memory used by the application and connections, not only epoll registrations.
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