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Date: Fri, 14 Jan 2022 14:38:16 +0100 From: Stefano Garzarella <sgarzare@...hat.com> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org, stefanha@...hat.com, kvm@...r.kernel.org, netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com> Subject: Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify() On Fri, Jan 14, 2022 at 07:45:35AM -0500, Michael S. Tsirkin wrote: >On Fri, Jan 14, 2022 at 10:05:08AM +0100, Stefano Garzarella wrote: >> In vhost_enable_notify() we enable the notifications and we read >> the avail index to check if new buffers have become available in >> the meantime. >> >> We are not caching the avail index, so when the device will call >> vhost_get_vq_desc(), it will find the old value in the cache and >> it will read the avail index again. >> >> It would be better to refresh the cache every time we read avail >> index, so let's change vhost_enable_notify() caching the value in >> `avail_idx` and compare it with `last_avail_idx` to check if there >> are new buffers available. >> >> Anyway, we don't expect a significant performance boost because >> the above path is not very common, indeed vhost_enable_notify() >> is often called with unlikely(), expecting that avail index has >> not been updated. >> >> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com> > >... and can in theory even hurt due to an extra memory write. >So ... performance test restults pls? Right, could be. I'll run some perf test with vsock, about net, do you have a test suite or common step to follow to test it? Thanks, Stefano
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