On Wed, Nov 10, 2021 at 07:53:49AM -0500, Michael S. Tsirkin wrote: > On Mon, Nov 08, 2021 at 10:47:40PM +0800, Xuan Zhuo wrote: > > On Mon, 8 Nov 2021 08:49:27 -0500, Michael S. Tsirkin <m...@redhat.com> > > wrote: > > > > > > Hmm a bunch of comments got ignored. See e.g. > > > https://lore.kernel.org/r/20211027043851-mutt-send-email-mst%40kernel.org > > > if they aren't relevant add code comments or commit log text explaining > > > the > > > design choice please. > > > > I should have responded to related questions, I am guessing whether some > > emails > > have been lost. > > > > I have sorted out the following 6 questions, if there are any missing > > questions, > > please let me know. > > > > 1. use list_head > > In the earliest version, I used pointers directly. You suggest that I use > > llist_head, but considering that llist_head has atomic operations. There > > is no > > competition problem here, so I used list_head. > > > > In fact, I did not increase the allocated space for list_head. > > > > use as desc array: | vring_desc | vring_desc | vring_desc | vring_desc | > > use as queue item: | list_head ........................................| > > the concern is that you touch many cache lines when removing an entry. > > I suggest something like: > > llist: add a non-atomic list_del_first > > One has to know what one's doing, but if one has locked the list > preventing all accesses, then it's ok to just pop off an entry without > atomics. > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > > --- > > diff --git a/include/linux/llist.h b/include/linux/llist.h > index 24f207b0190b..13a47dddb12b 100644 > --- a/include/linux/llist.h > +++ b/include/linux/llist.h > @@ -247,6 +247,17 @@ static inline struct llist_node *__llist_del_all(struct > llist_head *head) > > extern struct llist_node *llist_del_first(struct llist_head *head); > > +static inline struct llist_node *__llist_del_first(struct llist_head *head) > +{ > + struct llist_node *first = head->first; > + > + if (!first) > + return NULL; > + > + head->first = first->next; > + return first; > +} > + > struct llist_node *llist_reverse_order(struct llist_node *head); > > #endif /* LLIST_H */ > > > ----- > > > > 2. > > > > + if (vq->use_desc_cache && total_sg <= > > > > VIRT_QUEUE_CACHE_DESC_NUM) { > > > > + if (vq->desc_cache_chain) { > > > > + desc = vq->desc_cache_chain; > > > > + vq->desc_cache_chain = (void *)desc->addr; > > > > + goto got; > > > > + } > > > > + n = VIRT_QUEUE_CACHE_DESC_NUM; > > > > > > Hmm. This will allocate more entries than actually used. Why do it? > > > > > > This is because the size of each cache item is fixed, and the logic has been > > modified in the latest code. I think this problem no longer exists. > > > > > > 3. > > > What bothers me here is what happens if cache gets > > > filled on one numa node, then used on another? > > > > I'm thinking about another question, how did the cross-numa appear here, and > > virtio desc queue also has the problem of cross-numa. So is it necessary > > for us > > to deal with the cross-numa scene? > > It's true that desc queue might be cross numa, and people are looking > for ways to improve that. Not a reason to make things worse ... >
To add to that, given it's a concern, how about actually testing performance for this config? > > Indirect desc is used as virtio desc, so as long as it is in the same numa > > as > > virito desc. So we can allocate indirect desc cache at the same time when > > allocating virtio desc queue. > > Using it from current node like we do now seems better. > > > 4. > > > So e.g. for rx, we are wasting memory since indirect isn't used. > > > > In the current version, desc cache is set up based on pre-queue. > > > > So if the desc cache is not used, we don't need to set the desc cache. > > > > For example, virtio-net, as long as the tx queue and the rx queue in big > > packet > > mode enable desc cache. > > > I liked how in older versions adding indrect enabled it implicitly > though without need to hack drivers. > > > 5. > > > Would a better API be a cache size in bytes? This controls how much > > > memory is spent after all. > > > > My design is to set a threshold. When total_sg is greater than this > > threshold, > > it will fall back to kmalloc/kfree. When total_sg is less than or equal to > > this threshold, use the allocated cache. > > > > I know. My question is this, do devices know what a good threshold is? > If yes how do they know? > > > 6. kmem_cache_* > > > > I have tested these, the performance is not as good as the method used in > > this > > patch. > > Do you mean kmem_cache_alloc_bulk/kmem_cache_free_bulk? > You mentioned just kmem_cache_alloc previously. > > > > > Thanks. _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
