Re: [dpdk-dev] [PATCH v2 2/2] vhost: cache gpa to hpa translation

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Marvin,

On 4/1/20 4:50 PM, Marvin Liu wrote:
> If Tx zero copy enabled, gpa to hpa mapping table is updated one by
> one. This will harm performance when guest memory backend using 2M
> hugepages. Now add cached mapping table which will sorted by using
> sequence. Address translation will first check cached mapping table,
> then check unsorted mapping table if no match found.
> 
> Signed-off-by: Marvin Liu <yong.liu@intel.com>
> 

I don't like the approach, as I think it could have nasty effects.
For example, the system is loaded normally and let's say 25% of the
pages are used. Then we have a small spike, and buffers that were never
used start to be used, it will cause writing new entries into the cache
in the hot path when it is already overloaded. Wouldn't it increase the
number of packets dropped?

At set_mem_table time, instead of adding the guest pages unsorted, maybe
better to add them sorted there. Then you can use a better algorithm
than linear searching (O(n)), like binary search (O(log n)).

Thanks,
Maxime