linux-kernel - Re: [PATCH RFC v1 0/2] vhost: ring format independence

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Message-ID: <f650ac1a-6e2a-9215-6e4f-a1095f4a89cd@redhat.com>
Date:   Sat, 12 Oct 2019 16:15:42 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC v1 0/2] vhost: ring format independence


On 2019/10/11 下午9:45, Michael S. Tsirkin wrote:
> So the idea is as follows: we convert descriptors to an
> independent format first, and process that converting to
> iov later.
>
> The point is that we have a tight loop that fetches
> descriptors, which is good for cache utilization.
> This will also allow all kind of batching tricks -
> e.g. it seems possible to keep SMAP disabled while
> we are fetching multiple descriptors.


I wonder this may help for performance:

- another indirection layer, increased footprint

- won't help or even degrade when there's no batch

- an extra overhead in the case of in order where we should already had 
tight loop

- need carefully deal with indirect and chain or make it only work for 
packet sit just in a single descriptor

Thanks


>
> And perhaps more importantly, this is a very good fit for the packed
> ring layout, where we get and put descriptors in order.
>
> This patchset seems to already perform exactly the same as the original
> code already based on a microbenchmark.  More testing would be very much
> appreciated.
>
> Biggest TODO before this first step is ready to go in is to
> batch indirect descriptors as well.
>
> Integrating into vhost-net is basically
> s/vhost_get_vq_desc/vhost_get_vq_desc_batch/ -
> or add a module parameter like I did in the test module.
>
>
>
> Michael S. Tsirkin (2):
>    vhost: option to fetch descriptors through an independent struct
>    vhost: batching fetches
>
>   drivers/vhost/test.c  |  19 ++-
>   drivers/vhost/vhost.c | 333 +++++++++++++++++++++++++++++++++++++++++-
>   drivers/vhost/vhost.h |  20 ++-
>   3 files changed, 365 insertions(+), 7 deletions(-)
>