On 7/21/21 8:57 PM, Jeff Layton wrote:
On Wed, 2021-07-21 at 19:54 +0800, Xiubo Li wrote:
On 7/21/21 7:23 PM, Jeff Layton wrote:
On Wed, 2021-07-21 at 16:27 +0800, xiubli@xxxxxxxxxx wrote:
From: Xiubo Li <xiubli@xxxxxxxxxx>
The delayed_work will be executed per 5 seconds, during this time
the cap_delay_list may accumulate thounsands of caps need to flush,
this will make the MDS's dispatch queue be full and need a very long
time to handle them. And if there has some other operations, likes
a rmdir request, it will be add in the tail of dispath queue and
need to wait for several or tens of seconds.
In client side we shouldn't queue to many of the cap requests and
flush them if there has more than 100 items.
Why 100? My inclination is to say NAK on this.
This just from my test that around 100 client_caps requests queued will
work fine in most cases, which won't take too long to handle. Some times
the client will send thousands of requests in a short time, that will be
a problem.
What may be a better approach is to figure out why we're holding on to
so many caps and trying to flush them all at once. Maybe if we were to
more aggressively flush sooner, we'd not end up with such a backlog?
Yeah, I am still working on this.
From my analysis, some fixing or improvement like this will help reduce it:
commit e64f44a884657358812e6c057957be546db03cbe
Author: Xiubo Li <xiubli@xxxxxxxxxx>
Date: Wed May 27 09:09:27 2020 -0400
ceph: skip checking caps when session reconnecting and releasing reqs
It make no sense to check the caps when reconnecting to mds. And
for the async dirop caps, they will be put by its _cb() function,
so when releasing the requests, it will make no sense too.
From the test result, it seems that some caps related requests were
still pending in the cap_delay_list even after the rmdir requests were
already sent out and finished for the previous directories. I am still
confirming this.
I'm not a fan of this sort of arbitrary throttling in order to alleviate
a scalability problem in the MDS. The cap delay logic is already
horrifically complex as well, and this just makes it worse. If the MDS
happens to be processing requests from a lot of clients, this may not
help at all.
Yeah, right, in that case the mds dispatch queue possibly will have more
in a short time.
URL: https://tracker.ceph.com/issues/51734
The core problem sounds like the MDS got a call and just took way too
long to get to it. Shouldn't we be addressing this in the MDS instead?
It sounds like it needs to do a better job of parallelizing cap flushes.
The problem is that almost all the requests need to acquire the global
'mds_lock', so the parallelizing won't improve much.
Yuck. That sounds like the root cause right there. For the record, I'm
against papering over that with client patches.
Currently all the client side requests will have the priority of
CEPH_MSG_PRIO_DEFAULT:
m->hdr.priority = cpu_to_le16(CEPH_MSG_PRIO_DEFAULT);
Not sure whether can we just make the cap flushing request a lower
priority ? If this won't introduce other potential issue, that will work
because in the MDS dispatch queue it will handle high priority requests
first.
Tough call. You might mark it for lower priority, but then someone calls
fsync() and suddenly you need it to finish sooner. I think that sort of
change might backfire in some cases.
Yeah, I am also worrying about this.
Usually when faced with this sort of issue, the right answer is to try
to work ahead of the "spikes" in activity so that you don't have quite
such a backlog when you do need to flush everything out. I'm not sure
how possible that is here.
One underlying issue may be that some of the client's background
activity is currently driven by arbitrary timers (e.g. the delayed_work
timer). When the timer pops, it suddenly has to do a bunch of work, when
some of it could have been done earlier so that the load was spread out.
Signed-off-by: Xiubo Li <xiubli@xxxxxxxxxx>
---
fs/ceph/caps.c | 21 ++++++++++++++++++++-
fs/ceph/mds_client.c | 3 ++-
fs/ceph/mds_client.h | 3 +++
3 files changed, 25 insertions(+), 2 deletions(-)
diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index 4b966c29d9b5..064865761d2b 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -507,6 +507,8 @@ static void __cap_set_timeouts(struct ceph_mds_client *mdsc,
static void __cap_delay_requeue(struct ceph_mds_client *mdsc,
struct ceph_inode_info *ci)
{
+ int num = 0;
+
dout("__cap_delay_requeue %p flags 0x%lx at %lu\n", &ci->vfs_inode,
ci->i_ceph_flags, ci->i_hold_caps_max);
if (!mdsc->stopping) {
@@ -515,12 +517,19 @@ static void __cap_delay_requeue(struct ceph_mds_client *mdsc,
if (ci->i_ceph_flags & CEPH_I_FLUSH)
goto no_change;
list_del_init(&ci->i_cap_delay_list);
+ mdsc->num_cap_delay--;
}
__cap_set_timeouts(mdsc, ci);
list_add_tail(&ci->i_cap_delay_list, &mdsc->cap_delay_list);
+ num = ++mdsc->num_cap_delay;
no_change:
spin_unlock(&mdsc->cap_delay_lock);
}
+
+ if (num > 100) {
+ flush_delayed_work(&mdsc->delayed_work);
+ schedule_delayed(mdsc);
+ }
}
/*
@@ -531,13 +540,23 @@ static void __cap_delay_requeue(struct ceph_mds_client *mdsc,
static void __cap_delay_requeue_front(struct ceph_mds_client *mdsc,
struct ceph_inode_info *ci)
{
+ int num;
+
dout("__cap_delay_requeue_front %p\n", &ci->vfs_inode);
spin_lock(&mdsc->cap_delay_lock);
ci->i_ceph_flags |= CEPH_I_FLUSH;
- if (!list_empty(&ci->i_cap_delay_list))
+ if (!list_empty(&ci->i_cap_delay_list)) {
list_del_init(&ci->i_cap_delay_list);
+ mdsc->num_cap_delay--;
+ }
list_add(&ci->i_cap_delay_list, &mdsc->cap_delay_list);
+ num = ++mdsc->num_cap_delay;
spin_unlock(&mdsc->cap_delay_lock);
+
+ if (num > 100) {
+ flush_delayed_work(&mdsc->delayed_work);
+ schedule_delayed(mdsc);
+ }
}
/*
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 79aa4ce3a388..14e44de05812 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -4514,7 +4514,7 @@ void inc_session_sequence(struct ceph_mds_session *s)
/*
* delayed work -- periodically trim expired leases, renew caps with mds
*/
-static void schedule_delayed(struct ceph_mds_client *mdsc)
+void schedule_delayed(struct ceph_mds_client *mdsc)
{
int delay = 5;
unsigned hz = round_jiffies_relative(HZ * delay);
@@ -4616,6 +4616,7 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
mdsc->request_tree = RB_ROOT;
INIT_DELAYED_WORK(&mdsc->delayed_work, delayed_work);
mdsc->last_renew_caps = jiffies;
+ mdsc->num_cap_delay = 0;
INIT_LIST_HEAD(&mdsc->cap_delay_list);
INIT_LIST_HEAD(&mdsc->cap_wait_list);
spin_lock_init(&mdsc->cap_delay_lock);
diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
index a7af09257382..b4289b8d23ec 100644
--- a/fs/ceph/mds_client.h
+++ b/fs/ceph/mds_client.h
@@ -423,6 +423,7 @@ struct ceph_mds_client {
struct rb_root request_tree; /* pending mds requests */
struct delayed_work delayed_work; /* delayed work */
unsigned long last_renew_caps; /* last time we renewed our caps */
+ unsigned long num_cap_delay; /* caps in the cap_delay_list */
struct list_head cap_delay_list; /* caps with delayed release */
spinlock_t cap_delay_lock; /* protects cap_delay_list */
struct list_head snap_flush_list; /* cap_snaps ready to flush */
@@ -568,6 +569,8 @@ extern int ceph_trim_caps(struct ceph_mds_client *mdsc,
struct ceph_mds_session *session,
int max_caps);
+extern void schedule_delayed(struct ceph_mds_client *mdsc);
+
static inline int ceph_wait_on_async_create(struct inode *inode)
{
struct ceph_inode_info *ci = ceph_inode(inode);
