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

Ming Lei
2024-02-07T02:43:52Z

On Tue, Feb 06, 2024 at 07:31:26PM +0100, Mikulas Patocka wrote:
>
>
> 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 :)

You can run test over null-blk, which is enough to cover this report or change,
given this patch is marked as "Fixes: ", we need to understand what it fixes.

>
> > 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.

Yes.

But the two kinds of caches have different properties, such as:

- instruction cache has lower miss rate
- instruction cache is read only

so I'd suggest to provide null-blk test result at least.

Thanks,
Ming