Re: [PATCH] block: use the __packed attribute only on architectures where it is efficient

[Mikulas Patocka <mpatocka@redhat.com>]

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