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fd. The read built-in command calls read(2) with a 1-byte buffer because newline characters need to be detected even on a byte stream which comes from a non-seekable file descriptor. Because of this, the following script calls >6,000 read(2) to show a 6KiB file: while read IN; do echo "$IN"; done < /COPYRIGHT When the input byte stream is seekable, it is possible to read a data block and then reposition the file pointer to where a newline character found. This change adds a small buffer to do this and reduces the number of read(2) calls. Theoretically, multiple built-in commands reading the same seekable byte stream in a single pipe chain can share the buffer. However, this change just makes a single invocation of the read built-in allocate a buffer and deallocate it every time for simplicity. Although this causes read(2) to read the same regions multiple times, the performance penalty should be small compared to the reduction of read(2) calls. Reviewed by: jilles MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D23747 |
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