Re: [PATCH net-next v2] xen-netback: make copy batch size configurable

[David Miller <davem@davemloft.net>]

From: Joao Martins <joao.m.martins@oracle.com>
Date: Thu, 21 Dec 2017 17:24:28 +0000

> Commit eb1723a29b9a ("xen-netback: refactor guest rx") refactored Rx
> handling and as a result decreased max grant copy ops from 4352 to 64.
> Before this commit it would drain the rx_queue (while there are
> enough slots in the ring to put packets) then copy to all pages and write
> responses on the ring. With the refactor we do almost the same albeit
> the last two steps are done every COPY_BATCH_SIZE (64) copies.
> 
> For big packets, the value of 64 means copying 3 packets best case scenario
> (17 copies) and worst-case only 1 packet (34 copies, i.e. if all frags
> plus head cross the 4k grant boundary) which could be the case when
> packets go from local backend process.
> 
> Instead of making it static to 64 grant copies, lets allow the user to
> select its value (while keeping the current as default) by introducing
> the `copy_batch_size` module parameter. This allows users to select
> the higher batches (i.e. for better throughput with big packets) as it
> was prior to the above mentioned commit.
> 
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
> Changes since v1:
>  * move rx_copy.{idx,op} reallocation to separate helper
>  Addressed Paul's comments:
>  * rename xenvif_copy_state#size field to batch_size
>  * argument `size` should be unsigned int
>  * vfree is safe with NULL
>  * realloc rx_copy.{idx,op} after copy op flush

I truly dislike things of this nature.

When you give the user a numerical value to set, they have to pick
something.  This in turn requires deep, weird, knowledge of how the
driver implements RX packet processing.

That's asbolutely unacceptable.  Can you imagine being an admin and
trying to figure out what random number to plug into this thing?

"maximum number of grant copies on RX"

I've been the networking maintainer for more than 2 decades and I
have no idea whatsoever what kind of value I might want to set
there.

Nobody should have to know this other than people working on the
driver.

Instead, the issue is that the driver can optimize for throughput
or something else (latency, RX packing, I don't know exactly what
it is, but you're keeping the default value so it has some merit
right?).  Therefore, what you need to export is a boolean which
is self describing.

"rx_optimize_throughput"

That's it.  And you, the smart person who knows what this grant
copy mumbo jumbo means, can pick a specific value to use for
high throughput.

Thank you.