Performance: use MSG_DONTWAIT instead of select

[schweikert]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @schweikert, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a targeted performance optimization within the receive_packet function. By leveraging the MSG_DONTWAIT flag directly with recvmsg() when a zero wait_time is specified, the code avoids an unnecessary select() system call. This change streamlines the process of checking for incoming packets in a non-blocking manner, potentially leading to reduced latency and improved CPU efficiency for such operations.

Highlights

• Performance Optimization: Replaced the use of select() with MSG_DONTWAIT for immediate socket reads when the wait_time parameter is zero, aiming to reduce overhead.
• Non-Blocking Reads: The change directly attempts to read from IPv4 and potentially IPv6 sockets using recvmsg() with MSG_DONTWAIT, bypassing the select() system call for non-blocking scenarios.

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[coveralls]

coverage: 88.152% (+0.03%) from 88.12%
when pulling 496de51 on push-ulumtvlmkxlt
into 685edae on develop.

[schweikert]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a performance optimization by using MSG_DONTWAIT with recvmsg when polling for packets with no wait time, which avoids the overhead of the select() system call. This is a good optimization. Additionally, the change fixes a bug in the timeout calculation for select(), where an incorrect conversion from nanoseconds to microseconds could lead to incorrect wait times. The implementation of both the optimization and the bug fix appears correct and improves both performance and correctness.

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src/fping.c (2809-2816)

The previous calculation for to.tv_usec was buggy. The + 1 could cause to.tv_usec to become 1,000,000. According to POSIX, the value of tv_usec must be in the range [0, 999999]. An invalid value could lead to undefined behavior or, on systems that normalize it, cause select() to wait for up to a second longer than intended. The new implementation correctly converts nanoseconds to a timeval struct, fixing this issue.