Re: [dpdk-dev] [PATCH] vhost: avoid concurrency when logging dirty pages

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

On 03/27/2018 03:15 AM, Tan, Jianfeng wrote:
> 
> 
>> -----Original Message-----
>> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
>> Sent: Wednesday, March 21, 2018 11:44 PM
>> To: dev@dpdk.org; Tan, Jianfeng; Bie, Tiwei; jfreimann@redhat.com
>> Cc: stable@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: stable@dpdk.org
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> 
> As you mentioned, the concern is if it affects the rate of convergence. Hope we could have some tests on that.

We tested internally with live migration during PVP testing, at a rate
of 3Mpps.
In these conditions, we obtain same numbers as if we revert
897f13a1f726.

> Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>

Thanks,
Maxime

> 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
>