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

[Liu, Yong]

> -----Original Message-----
> From: Gavin Hu <Gavin.Hu at arm.com>
> Sent: Thursday, April 2, 2020 11:05 AM
> To: Liu, Yong <yong.liu at intel.com>; maxime.coquelin at redhat.com; Ye,
> Xiaolong <xiaolong.ye at intel.com>; Wang, Zhihong
> <zhihong.wang at intel.com>
> Cc: dev at dpdk.org; nd <nd at arm.com>; nd <nd at arm.com>
> Subject: RE: [dpdk-dev] [PATCH v2 2/2] vhost: cache gpa to hpa translation
> 
> Hi Marvin,
> 
> > -----Original Message-----
> > From: Liu, Yong <yong.liu at intel.com>
> > Sent: Wednesday, April 1, 2020 9:01 PM
> > To: Gavin Hu <Gavin.Hu at arm.com>; maxime.coquelin at redhat.com; Ye,
> > Xiaolong <xiaolong.ye at intel.com>; Wang, Zhihong
> > <zhihong.wang at intel.com>
> > Cc: dev at dpdk.org; nd <nd at arm.com>
> > Subject: RE: [dpdk-dev] [PATCH v2 2/2] vhost: cache gpa to hpa translation
> >
> >
> >
> > > -----Original Message-----
> > > From: Gavin Hu <Gavin.Hu at arm.com>
> > > Sent: Wednesday, April 1, 2020 6:07 PM
> > > To: Liu, Yong <yong.liu at intel.com>; maxime.coquelin at redhat.com; Ye,
> > > Xiaolong <xiaolong.ye at intel.com>; Wang, Zhihong
> > > <zhihong.wang at intel.com>
> > > Cc: dev at dpdk.org; nd <nd at arm.com>
> > > Subject: RE: [dpdk-dev] [PATCH v2 2/2] vhost: cache gpa to hpa
> translation
> > >
> > > Hi Marvin,
> > >
> > > > -----Original Message-----
> > > > From: dev <dev-bounces at dpdk.org> On Behalf Of Marvin Liu
> > > > Sent: Wednesday, April 1, 2020 10:50 PM
> > > > To: maxime.coquelin at redhat.com; xiaolong.ye at intel.com;
> > > > zhihong.wang at intel.com
> > > > Cc: dev at dpdk.org; Marvin Liu <yong.liu at intel.com>
> > > > Subject: [dpdk-dev] [PATCH v2 2/2] vhost: cache gpa to hpa translation
> > > >
> > > > 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 at intel.com>
> > > >
> > > > diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> > > > index 2087d1400..5cb0e83dd 100644
> > > > --- a/lib/librte_vhost/vhost.h
> > > > +++ b/lib/librte_vhost/vhost.h
> > > > @@ -368,7 +368,9 @@ struct virtio_net {
> > > >  	struct vhost_device_ops const *notify_ops;
> > > >
> > > >  	uint32_t		nr_guest_pages;
> > > > +	uint32_t		nr_cached_guest_pages;
> > > >  	uint32_t		max_guest_pages;
> > > > +	struct guest_page       *cached_guest_pages;
> > > >  	struct guest_page       *guest_pages;
> > > >
> > > >  	int			slave_req_fd;
> > > > @@ -553,12 +555,25 @@ gpa_to_hpa(struct virtio_net *dev, uint64_t
> > gpa,
> > > > uint64_t size)
> > > >  {
> > > >  	uint32_t i;
> > > >  	struct guest_page *page;
> > > > +	uint32_t cached_pages = dev->nr_cached_guest_pages;
> > > > +
> 
> Add a comment here, something like "Firstly look up in the cached pages"?
> 
> > > > +	for (i = 0; i < cached_pages; i++) {
> 
> Should the searching order reversed  here to search the most recent entries?
> 
> > > > +		page = &dev->cached_guest_pages[i];
> > > > +		if (gpa >= page->guest_phys_addr &&
> > > > +			gpa + size < page->guest_phys_addr + page->size) {
> > > > +			return gpa - page->guest_phys_addr +
> > > > +				page->host_phys_addr;
> > > > +		}
> > > > +	}
> > > Sorry, I did not see any speedup with cached guest pages in comparison
> to
> > > the old code below.
> > > Is it not a simple copy?
> > > Is it a better idea to use hash instead to speed up the translation?
> > > /Gavin
> >
> > Hi Gavin,
> > Here just resort the overall mapping table according to using sequence.
> > Most likely virtio driver will reuse recently cycled buffers, thus search will
> > find match in beginning.
> > That is simple fix for performance enhancement. If use hash for index, it
> will
> > take much more cost in normal case.
> >
> > Regards,
> > Marvin
> 
> There are issues here, the cached table is growing over time, will it become
> less efficient when growing too big and even bigger than the original table
> and overflow happen?
> Is it a good idea to limit the cached entries to small therefore make the
> search quicker?
> /Gavin
> >

Gavin,
Cached table size is the same as mapping table, it only recorded entries from original table which have been used.
At worst case like every access of guest memory is random, cached table will be same size of original size. Search cost is same as before. 
It will be hard to predict which size is more suitable for caching, that is depend on how guest driver allocate buffer.  Maybe less than ten when using 2M page and thousands when using 4K page.
So here just add resorted table, which cost is much less in normal case and same as before at worst case. 

Thanks,
Marvin

> >
> > > >
> Add a comment here, something like "Fall back to normal lookup if failed to
> get from the cached table"?
> 
> > > >  	for (i = 0; i < dev->nr_guest_pages; i++) {
> > > >  		page = &dev->guest_pages[i];
> > > >
> > > >  		if (gpa >= page->guest_phys_addr &&
> > > >  		    gpa + size < page->guest_phys_addr + page->size) {
> > > > +			rte_memcpy(&dev-
> > > > >cached_guest_pages[cached_pages],
> > > > +				   page, sizeof(struct guest_page));
> > > > +			dev->nr_cached_guest_pages++;
> 
> Will overflow happen over time when there are many translations?
> 
> > > >  			return gpa - page->guest_phys_addr +
> > > >  			       page->host_phys_addr;
> > > >  		}
> > > > diff --git a/lib/librte_vhost/vhost_user.c
> b/lib/librte_vhost/vhost_user.c
> > > > index 79fcb9d19..1bae1fddc 100644
> > > > --- a/lib/librte_vhost/vhost_user.c
> > > > +++ b/lib/librte_vhost/vhost_user.c
> > > > @@ -192,7 +192,9 @@ vhost_backend_cleanup(struct virtio_net *dev)
> > > >  	}
> > > >
> > > >  	rte_free(dev->guest_pages);
> > > > +	rte_free(dev->cached_guest_pages);
> > > >  	dev->guest_pages = NULL;
> > > > +	dev->cached_guest_pages = NULL;
> > > >
> > > >  	if (dev->log_addr) {
> > > >  		munmap((void *)(uintptr_t)dev->log_addr, dev->log_size);
> > > > @@ -898,7 +900,7 @@ add_one_guest_page(struct virtio_net *dev,
> > > > uint64_t guest_phys_addr,
> > > >  		   uint64_t host_phys_addr, uint64_t size)
> > > >  {
> > > >  	struct guest_page *page, *last_page;
> > > > -	struct guest_page *old_pages;
> > > > +	struct guest_page *old_pages, *old_cached_pages;
> > > >
> > > >  	if (dev->nr_guest_pages == dev->max_guest_pages) {
> > > >  		dev->max_guest_pages *= 2;
> > > > @@ -906,9 +908,19 @@ add_one_guest_page(struct virtio_net *dev,
> > > > uint64_t guest_phys_addr,
> > > >  		dev->guest_pages = rte_realloc(dev->guest_pages,
> > > >  					dev->max_guest_pages *
> > > > sizeof(*page),
> > > >  					RTE_CACHE_LINE_SIZE);
> > > > -		if (dev->guest_pages == NULL) {
> > > > +		old_cached_pages = dev->cached_guest_pages;
> > > > +		dev->cached_guest_pages = rte_realloc(dev-
> > > > >cached_guest_pages,
> > > > +						dev->max_guest_pages *
> > > > +						sizeof(*page),
> > > > +						RTE_CACHE_LINE_SIZE);
> > > > +		dev->nr_cached_guest_pages = 0;
> > > > +		if (dev->guest_pages == NULL ||
> > > > +				dev->cached_guest_pages == NULL) {
> > > >  			VHOST_LOG_CONFIG(ERR, "cannot realloc
> > > > guest_pages\n");
> > > >  			rte_free(old_pages);
> > > > +			rte_free(old_cached_pages);
> > > > +			dev->guest_pages = NULL;
> > > > +			dev->cached_guest_pages = NULL;
> > > >  			return -1;
> > > >  		}
> > > >  	}
> > > > @@ -1078,6 +1090,20 @@ vhost_user_set_mem_table(struct
> virtio_net
> > > > **pdev, struct VhostUserMsg *msg,
> > > >  		}
> > > >  	}
> > > >
> > > > +	if (dev->cached_guest_pages == NULL) {
> > > > +		dev->cached_guest_pages = rte_zmalloc(NULL,
> > > > +						dev->max_guest_pages *
> > > > +						sizeof(struct guest_page),
> > > > +						RTE_CACHE_LINE_SIZE);
> > > > +		if (dev->cached_guest_pages == NULL) {
> > > > +			VHOST_LOG_CONFIG(ERR,
> > > > +				"(%d) failed to allocate memory "
> > > > +				"for dev->cached_guest_pages\n",
> > > > +				dev->vid);
> > > > +			return RTE_VHOST_MSG_RESULT_ERR;
> > > > +		}
> > > > +	}
> > > > +
> > > >  	dev->mem = rte_zmalloc("vhost-mem-table", sizeof(struct
> > > > rte_vhost_memory) +
> > > >  		sizeof(struct rte_vhost_mem_region) * memory->nregions,
> > > > 0);
> > > >  	if (dev->mem == NULL) {
> > > > --
> > > > 2.17.1