On Tue, Feb 06, 2024 at 12:14:14PM +0100, Mikulas Patocka wrote: > The __packed macro (expanding to __attribute__((__packed__))) specifies > that the structure has an alignment of 1. Therefore, it may be arbitrarily > misaligned. On architectures that don't have hardware support for > unaligned accesses, gcc generates very inefficient code that accesses the > structure fields byte-by-byte and assembles the result using shifts and > ors. > > For example, on PA-RISC, this function is compiled to 23 instructions with > the __packed attribute and only 2 instructions without the __packed > attribute. Can you share user visible effects in this way? such as IOPS or CPU utilization effect when running typical workload on null_blk or NVMe. CPU is supposed to be super fast if the data is in single L1 cacheline, but removing '__packed' may introduce one extra L1 cacheline load for bio. thanks, Ming
