On Tue, 6 Feb 2024, Ming Lei wrote: > 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. The patch reduces total kernel size by 4096 bytes. The parisc machine doesn't have PCIe, so I can't test it with 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. Saving the intruction cache is also important. Removing the __packed keyword increases the bio structure size by 8 bytes - that is, L1 data cache consumption will be increased with the probability 8/64. And it reduces L1 instruction cache consumption by 84 bytes - that is one or two cachelines. Mikulas
