Re: RFC: overheads of using mb_unmapped_to_ext?

> On May 10, 2026, at 6:47 AM, Rick Macklem <[email protected]> wrote:
> 
> On Sat, May 9, 2026 at 3:43 PM Rick Macklem <[email protected]> wrote:
>> 
>> Hi,
>> 
>> I have a simple patch that makes the NFS server generate
>> read replies as a list of m_extpg mbufs. (The code already
>> knows how to do this for the KTLS, but this patch enables
>> it for non-TLS use.)
>> 
>> I am wondering if the overhead of calling
>> mb_unmapped_to_ext() on the mbuf chain when the
>> NIC does not have IFCAP_MEXTPG makes doing
>> this an undesirable alternative to generating a longer
>> mbuf chain of mbuf clusters?
>> 
>> For my hardware, things just run at wire speed (1Gbps),
>> so I do not see any difference between the two cases.
>> I do see a 15% improvement for localhost mounts, but
>> those hardly matter.
>> 
>> I tester did determine that performance was improved for
>> a Mellanox NIC that has IFCAP_MEXTPG. (5-15% depending
>> on setup)
>> 
>> I can enable this two ways:
>> 1 - A sysctl that is enabled/disabled by default (which means most
>>     cases will call mb_unmapped_to_ext() if enabled).
>> or
>> 2 - Try and check to see if the outbound NIC has IFCAP_MEXTPG.
>>    (Note that this does not have to be 100% correct, since the NFS
>>     reply works either way.  The reviewer was concerned about routing
>>     changes, etc, affecting the result.)
>> 
>> I implemented #2 using the suggested approach, but the
>> reviewer (who suggested the approach) decided that doing so
>> was not recommended. (D55203 on phabricator.)
>> 
>> So, what do you think is the preferred alternative?
> I may have partially answered my own question.
> I did a crude measurement of cpu overheads and found that,
> for a simple amd64 system with a 1Gbps Intel NIC, that the
> overheads appear to be about 10% better (aka lower) when
> M_EXTPG mbufs are used.

I think the overheads is acceptable on slow NICs.

For the 1st approach, if a user find performance regression, he / she 
can just tune sysctl to turn off mb_unmapped_to_ext() . That is much
simple and robust, and easy to maintain.

> 
> So, unless others think there are cases where using M_EXTPG
> mbufs for read replies is undesirable, I think enabling them by
> default regardless of the NIC, might be ok.
> 
> rick
> 
>> 
>> rick
> 

Best regards,
Zhenlei