> -----Original Message-----
> From: Maxime Coquelin [mailto:maxime.coque...@redhat.com]
> Sent: Wednesday, March 21, 2018 11:44 PM
> To: dev@dpdk.org; Tan, Jianfeng; Bie, Tiwei; jfreim...@redhat.com
> Cc: sta...@dpdk.org; Maxime Coquelin
> Subject: [PATCH] vhost: avoid concurrency when logging dirty pages
>
> This patch aims at fixing a migration performance regression
> faced since atomic operation is used to log pages as dirty when
> doing live migration.
>
> Instead of setting a single bit by doing an atomic read-modify-write
> operation to log a page as dirty, this patch write 0xFF to the
> corresponding byte, and so logs 8 page as dirty.
>
> The advantage is that it avoids concurrent atomic operations by
> multiple PMD threads, the drawback is that some clean pages are
> marked as dirty and so are transferred twice.
>
> Fixes: 897f13a1f726 ("vhost: make page logging atomic")
> Cc: sta...@dpdk.org
>
> Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
As you mentioned, the concern is if it affects the rate of convergence. Hope we
could have some tests on that.
Reviewed-by: Jianfeng Tan <jianfeng....@intel.com>
Thanks!
> ---
> lib/librte_vhost/vhost.h | 12 ++++--------
> 1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> index d947bc9e3..aa2606f8a 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -247,18 +247,14 @@ struct virtio_net {
> #define VHOST_LOG_PAGE 4096
>
> /*
> - * Atomically set a bit in memory.
> + * Mark all pages belonging to the same dirty log bitmap byte
> + * as dirty. The goal is to avoid concurrency between different
> + * threads doing atomic read-modify-writes on the same byte.
> */
> -static __rte_always_inline void
> -vhost_set_bit(unsigned int nr, volatile uint8_t *addr)
> -{
> - __sync_fetch_and_or_8(addr, (1U << nr));
> -}
> -
> static __rte_always_inline void
> vhost_log_page(uint8_t *log_base, uint64_t page)
> {
> - vhost_set_bit(page % 8, &log_base[page / 8]);
> + log_base[page / 8] = 0xff;
> }
>
> static __rte_always_inline void
> --
> 2.14.3
