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drbd: add back some fairness to AL transactions

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When batching more updates to the activity log into single transactions,
we lost the ability for new requests to force themselves into the active
set: all preparation steps became non-blocking, and if all currently
hot extents keep busy, they could starve out new incoming requests
to cold extents for quite a while.

This can only happen if your IO backend accepts more IO operations per
average DRBD replication round trip time than you have al-extents
configured.

If we have incoming requests to cold extents,
at least do one blocking update per transaction.

In an artificial worst-case workload on SSD with an asynchronous 600 ms
replication link, with al-extents = 7 (the minimum we allow), and
concurrent full resynch, without this patch, some write requests have
been observed to be starved for 40 seconds.
With this patch, application observed a worst case latency of twice the
replication round trip time.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Lars Ellenberg 2014-04-28 18:43:28 +02:00 committed by Jens Axboe
parent fa090e708a
commit e4d7d6f4d3
3 changed files with 22 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
drivers/block/drbd/drbd_actlog.c
View File

@ -203,7 +203,7 @@ int drbd_md_sync_page_io(struct drbd_device *device, struct drbd_backing_dev *bd
BUG_ON(!bdev->md_bdev);
drbd_dbg(device, "meta_data io: %s [%d]:%s(,%llus,%s) %pS\n",
dynamic_drbd_dbg(device, "meta_data io: %s [%d]:%s(,%llus,%s) %pS\n",
current->comm, current->pid, __func__,
(unsigned long long)sector, (rw & WRITE) ? "WRITE" : "READ",
(void*)_RET_IP_ );
@ -275,7 +275,6 @@ bool drbd_al_begin_io_fastpath(struct drbd_device *device, struct drbd_interval
return _al_get(device, first, true);
}
static
bool drbd_al_begin_io_prepare(struct drbd_device *device, struct drbd_interval *i)
{
/* for bios crossing activity log extent boundaries,

1
drivers/block/drbd/drbd_int.h
View File

@ -1487,6 +1487,7 @@ extern const char *drbd_conn_str(enum drbd_conns s);
extern const char *drbd_role_str(enum drbd_role s);
/* drbd_actlog.c */
extern bool drbd_al_begin_io_prepare(struct drbd_device *device, struct drbd_interval *i);
extern int drbd_al_begin_io_nonblock(struct drbd_device *device, struct drbd_interval *i);
extern void drbd_al_begin_io_commit(struct drbd_device *device, bool delegate);
extern bool drbd_al_begin_io_fastpath(struct drbd_device *device, struct drbd_interval *i);

20
drivers/block/drbd/drbd_req.c
View File

@ -1242,6 +1242,7 @@ void do_submit(struct work_struct *ws)
if (list_empty(&incoming))
break;
skip_fast_path:
wait_event(device->al_wait, prepare_al_transaction_nonblock(device, &incoming, &pending));
/* Maybe more was queued, while we prepared the transaction?
* Try to stuff them into this transaction as well.
@ -1280,6 +1281,25 @@ void do_submit(struct work_struct *ws)
list_del_init(&req->tl_requests);
drbd_send_and_submit(device, req);
}
/* If all currently hot activity log extents are kept busy by
* incoming requests, we still must not totally starve new
* requests to cold extents. In that case, prepare one request
* in blocking mode. */
list_for_each_entry_safe(req, tmp, &incoming, tl_requests) {
list_del_init(&req->tl_requests);
req->rq_state |= RQ_IN_ACT_LOG;
if (!drbd_al_begin_io_prepare(device, &req->i)) {
/* Corresponding extent was hot after all? */
drbd_send_and_submit(device, req);
} else {
/* Found a request to a cold extent.
* Put on "pending" list,
* and try to cumulate with more. */
list_add(&req->tl_requests, &pending);
goto skip_fast_path;
}
}
}
}