Hi Coly,
Hello Coly,
> On 2018/4/12 2:38 PM, tang.junhui@xxxxxxxxxx wrote:
> > From: Tang Junhui <tang.junhui@xxxxxxxxxx>
> >
> > In GC thread, we record the latest GC key in gc_done, which is expected
> > to be used for incremental GC, but in currently code, we didn't realize
> > it. When GC runs, front side IO would be blocked until the GC over, it
> > would be a long time if there is a lot of btree nodes.
> >
> > This patch realizes incremental GC, the main ideal is that, when there
> > are front side I/Os, after GC some nodes (100), we stop GC, release locker
> > of the btree node, and go to process the front side I/Os for some times
> > (100 ms), then go back to GC again.
> >
> > By this patch, when we doing GC, I/Os are not blocked all the time, and
> > there is no obvious I/Os zero jump problem any more.
> >
> > Signed-off-by: Tang Junhui <tang.junhui@xxxxxxxxxx>
>
> Hi Junhui,
>
> I reply my comments in line,
Thanks for your comments in advance.
>
> > ---
> > drivers/md/bcache/bcache.h | 5 +++++
> > drivers/md/bcache/btree.c | 15 ++++++++++++++-
> > drivers/md/bcache/request.c | 3 +++
> > 3 files changed, 22 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> > index 843877e..ab4c9ca 100644
> > --- a/drivers/md/bcache/bcache.h
> > +++ b/drivers/md/bcache/bcache.h
> > @@ -445,6 +445,7 @@ struct cache {
> >
> > struct gc_stat {
> > size_t nodes;
> > + size_t nodes_pre;
> > size_t key_bytes;
> >
> > size_t nkeys;
> > @@ -568,6 +569,10 @@ struct cache_set {
> > */
> > atomic_t rescale;
> > /*
> > + * used for GC, identify if any front side I/Os is inflight
> > + */
> > + atomic_t io_inflight;
>
> Could you please to rename the above variable to something like
> search_inflight ? Just to be more explicit, considering we have many
> types of io requests.
OK, It looks better.
>
> > + /*
> > * When we invalidate buckets, we use both the priority and the amount
> > * of good data to determine which buckets to reuse first - to weight
> > * those together consistently we keep track of the smallest nonzero
> > diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> > index 81e8dc3..b36d276 100644
> > --- a/drivers/md/bcache/btree.c
> > +++ b/drivers/md/bcache/btree.c
> > @@ -90,6 +90,9 @@
> >
> > #define MAX_NEED_GC 64
> > #define MAX_SAVE_PRIO 72
> > +#define MIN_GC_NODES 100
> > +#define GC_SLEEP_TIME 100
>
> How about naming the above macro as GC_SLEEP_MS ? So people may
> understand the sleep time is 100ms without checking more context.
OK, It looks better.
> > +
> >
> > #define PTR_DIRTY_BIT (((uint64_t) 1 << 36))
> >
> > @@ -1581,6 +1584,13 @@ static int btree_gc_recurse(struct btree *b, struct btree_op *op,
> > memmove(r + 1, r, sizeof(r[0]) * (GC_MERGE_NODES - 1));
> > r->b = NULL;
> >
> > + if (atomic_read(&b->c->io_inflight) &&
> > + gc->nodes >= gc->nodes_pre + MIN_GC_NODES) {
>
> On 32bit machines, gc->nodes is a 32bit count, if it is overflowed the
> above check will be false for a very long time, and in some condition it
> might be always false after gc->nodes overflowed.
>
> How about adding the following check before the if() statement ?
> /* in case gc->nodes is overflowed */
> if (gc->nodes_pre < gc->nodes)
> gc->noeds_pre = gc->nodes;
>
I think 32bit is big enough, which can express a value of billions,
but the number of nodes is usually around tens of thousands.
Also gc->nodes and gc->nodes_pre were set to zero when GC begin each time.
How do you think about?
> > + gc->nodes_pre = gc->nodes;
> > + ret = -EAGAIN;
> > + break;
> > + }
> > +
> > if (need_resched()) {
> > ret = -EAGAIN;
> > break;
> > @@ -1748,7 +1758,10 @@ static void bch_btree_gc(struct cache_set *c)
> > closure_sync(&writes);
> > cond_resched();
> >
> > - if (ret && ret != -EAGAIN)
> > + if (ret == -EAGAIN)
> > + schedule_timeout_interruptible(msecs_to_jiffies
> > + (GC_SLEEP_TIME));
> > + else if (ret)
> > pr_warn("gc failed!");
> > } while (ret);
> >
> > diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
> > index 643c3021..26750a9 100644
> > --- a/drivers/md/bcache/request.c
> > +++ b/drivers/md/bcache/request.c
> > @@ -637,7 +637,9 @@ static void do_bio_hook(struct search *s, struct bio *orig_bio)
> > static void search_free(struct closure *cl)
> > {
> > struct search *s = container_of(cl, struct search, cl);
> > +
> > bio_complete(s);
> > + atomic_dec(&s->d->c->io_inflight);
> >
> > if (s->iop.bio)
> > bio_put(s->iop.bio);
> > @@ -655,6 +657,7 @@ static inline struct search *search_alloc(struct bio *bio,
> >
> > closure_init(&s->cl, NULL);
> > do_bio_hook(s, bio);
> > + atomic_inc(&d->c->io_inflight);
> >
>
> Similar variable naming.
>
> > s->orig_bio = bio;
> > s->cache_miss = NULL;
> >
>
Thanks for your comments.
Tang Junhui
