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824e47dadd
When I run a parallel reading performan testing on a md raid1 device with two NVMe SSDs, I observe very bad throughput in supprise: by fio with 64KB block size, 40 seq read I/O jobs, 128 iodepth, overall throughput is only 2.7GB/s, this is around 50% of the idea performance number. The perf reports locking contention happens at allow_barrier() and wait_barrier() code, - 41.41% fio [kernel.kallsyms] [k] _raw_spin_lock_irqsave - _raw_spin_lock_irqsave + 89.92% allow_barrier + 9.34% __wake_up - 37.30% fio [kernel.kallsyms] [k] _raw_spin_lock_irq - _raw_spin_lock_irq - 100.00% wait_barrier The reason is, in these I/O barrier related functions, - raise_barrier() - lower_barrier() - wait_barrier() - allow_barrier() They always hold conf->resync_lock firstly, even there are only regular reading I/Os and no resync I/O at all. This is a huge performance penalty. The solution is a lockless-like algorithm in I/O barrier code, and only holding conf->resync_lock when it has to. The original idea is from Hannes Reinecke, and Neil Brown provides comments to improve it. I continue to work on it, and make the patch into current form. In the new simpler raid1 I/O barrier implementation, there are two wait barrier functions, - wait_barrier() Which calls _wait_barrier(), is used for regular write I/O. If there is resync I/O happening on the same I/O barrier bucket, or the whole array is frozen, task will wait until no barrier on same barrier bucket, or the whold array is unfreezed. - wait_read_barrier() Since regular read I/O won't interfere with resync I/O (read_balance() will make sure only uptodate data will be read out), it is unnecessary to wait for barrier in regular read I/Os, waiting in only necessary when the whole array is frozen. The operations on conf->nr_pending[idx], conf->nr_waiting[idx], conf-> barrier[idx] are very carefully designed in raise_barrier(), lower_barrier(), _wait_barrier() and wait_read_barrier(), in order to avoid unnecessary spin locks in these functions. Once conf-> nr_pengding[idx] is increased, a resync I/O with same barrier bucket index has to wait in raise_barrier(). Then in _wait_barrier() if no barrier raised in same barrier bucket index and array is not frozen, the regular I/O doesn't need to hold conf->resync_lock, it can just increase conf->nr_pending[idx], and return to its caller. wait_read_barrier() is very similar to _wait_barrier(), the only difference is it only waits when array is frozen. For heavy parallel reading I/Os, the lockless I/O barrier code almostly gets rid of all spin lock cost. This patch significantly improves raid1 reading peroformance. From my testing, a raid1 device built by two NVMe SSD, runs fio with 64KB blocksize, 40 seq read I/O jobs, 128 iodepth, overall throughput increases from 2.7GB/s to 4.6GB/s (+70%). Changelog V4: - Change conf->nr_queued[] to atomic_t. - Define BARRIER_BUCKETS_NR_BITS by (PAGE_SHIFT - ilog2(sizeof(atomic_t))) V3: - Add smp_mb__after_atomic() as Shaohua and Neil suggested. - Change conf->nr_queued[] from atomic_t to int. - Change conf->array_frozen from atomic_t back to int, and use READ_ONCE(conf->array_frozen) to check value of conf->array_frozen in _wait_barrier() and wait_read_barrier(). - In _wait_barrier() and wait_read_barrier(), add a call to wake_up(&conf->wait_barrier) after atomic_dec(&conf->nr_pending[idx]), to fix a deadlock between _wait_barrier()/wait_read_barrier and freeze_array(). V2: - Remove a spin_lock/unlock pair in raid1d(). - Add more code comments to explain why there is no racy when checking two atomic_t variables at same time. V1: - Original RFC patch for comments. Signed-off-by: Coly Li <colyli@suse.de> Cc: Shaohua Li <shli@fb.com> Cc: Hannes Reinecke <hare@suse.com> Cc: Johannes Thumshirn <jthumshirn@suse.de> Cc: Guoqing Jiang <gqjiang@suse.com> Reviewed-by: Neil Brown <neilb@suse.de> Signed-off-by: Shaohua Li <shli@fb.com>
199 lines
5.6 KiB
C
199 lines
5.6 KiB
C
#ifndef _RAID1_H
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#define _RAID1_H
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/*
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* each barrier unit size is 64MB fow now
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* note: it must be larger than RESYNC_DEPTH
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*/
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#define BARRIER_UNIT_SECTOR_BITS 17
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#define BARRIER_UNIT_SECTOR_SIZE (1<<17)
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/*
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* In struct r1conf, the following members are related to I/O barrier
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* buckets,
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* atomic_t *nr_pending;
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* atomic_t *nr_waiting;
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* atomic_t *nr_queued;
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* atomic_t *barrier;
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* Each of them points to array of atomic_t variables, each array is
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* designed to have BARRIER_BUCKETS_NR elements and occupy a single
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* memory page. The data width of atomic_t variables is 4 bytes, equal
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* to 1<<(ilog2(sizeof(atomic_t))), BARRIER_BUCKETS_NR_BITS is defined
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* as (PAGE_SHIFT - ilog2(sizeof(int))) to make sure an array of
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* atomic_t variables with BARRIER_BUCKETS_NR elements just exactly
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* occupies a single memory page.
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*/
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#define BARRIER_BUCKETS_NR_BITS (PAGE_SHIFT - ilog2(sizeof(atomic_t)))
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#define BARRIER_BUCKETS_NR (1<<BARRIER_BUCKETS_NR_BITS)
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struct raid1_info {
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struct md_rdev *rdev;
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sector_t head_position;
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/* When choose the best device for a read (read_balance())
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* we try to keep sequential reads one the same device
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*/
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sector_t next_seq_sect;
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sector_t seq_start;
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};
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/*
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* memory pools need a pointer to the mddev, so they can force an unplug
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* when memory is tight, and a count of the number of drives that the
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* pool was allocated for, so they know how much to allocate and free.
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* mddev->raid_disks cannot be used, as it can change while a pool is active
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* These two datums are stored in a kmalloced struct.
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* The 'raid_disks' here is twice the raid_disks in r1conf.
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* This allows space for each 'real' device can have a replacement in the
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* second half of the array.
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*/
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struct pool_info {
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struct mddev *mddev;
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int raid_disks;
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};
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struct r1conf {
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struct mddev *mddev;
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struct raid1_info *mirrors; /* twice 'raid_disks' to
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* allow for replacements.
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*/
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int raid_disks;
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spinlock_t device_lock;
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/* list of 'struct r1bio' that need to be processed by raid1d,
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* whether to retry a read, writeout a resync or recovery
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* block, or anything else.
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*/
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struct list_head retry_list;
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/* A separate list of r1bio which just need raid_end_bio_io called.
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* This mustn't happen for writes which had any errors if the superblock
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* needs to be written.
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*/
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struct list_head bio_end_io_list;
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/* queue pending writes to be submitted on unplug */
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struct bio_list pending_bio_list;
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int pending_count;
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/* for use when syncing mirrors:
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* We don't allow both normal IO and resync/recovery IO at
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* the same time - resync/recovery can only happen when there
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* is no other IO. So when either is active, the other has to wait.
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* See more details description in raid1.c near raise_barrier().
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*/
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wait_queue_head_t wait_barrier;
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spinlock_t resync_lock;
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atomic_t *nr_pending;
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atomic_t *nr_waiting;
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atomic_t *nr_queued;
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atomic_t *barrier;
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int array_frozen;
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/* Set to 1 if a full sync is needed, (fresh device added).
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* Cleared when a sync completes.
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*/
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int fullsync;
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/* When the same as mddev->recovery_disabled we don't allow
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* recovery to be attempted as we expect a read error.
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*/
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int recovery_disabled;
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/* poolinfo contains information about the content of the
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* mempools - it changes when the array grows or shrinks
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*/
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struct pool_info *poolinfo;
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mempool_t *r1bio_pool;
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mempool_t *r1buf_pool;
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/* temporary buffer to synchronous IO when attempting to repair
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* a read error.
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*/
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struct page *tmppage;
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/* When taking over an array from a different personality, we store
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* the new thread here until we fully activate the array.
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*/
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struct md_thread *thread;
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/* Keep track of cluster resync window to send to other
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* nodes.
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*/
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sector_t cluster_sync_low;
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sector_t cluster_sync_high;
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};
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/*
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* this is our 'private' RAID1 bio.
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*
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* it contains information about what kind of IO operations were started
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* for this RAID1 operation, and about their status:
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*/
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struct r1bio {
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atomic_t remaining; /* 'have we finished' count,
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* used from IRQ handlers
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*/
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atomic_t behind_remaining; /* number of write-behind ios remaining
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* in this BehindIO request
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*/
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sector_t sector;
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int sectors;
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unsigned long state;
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struct mddev *mddev;
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/*
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* original bio going to /dev/mdx
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*/
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struct bio *master_bio;
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/*
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* if the IO is in READ direction, then this is where we read
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*/
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int read_disk;
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struct list_head retry_list;
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/* Next two are only valid when R1BIO_BehindIO is set */
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struct bio_vec *behind_bvecs;
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int behind_page_count;
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/*
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* if the IO is in WRITE direction, then multiple bios are used.
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* We choose the number when they are allocated.
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*/
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struct bio *bios[0];
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/* DO NOT PUT ANY NEW FIELDS HERE - bios array is contiguously alloced*/
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};
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/* bits for r1bio.state */
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enum r1bio_state {
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R1BIO_Uptodate,
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R1BIO_IsSync,
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R1BIO_Degraded,
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R1BIO_BehindIO,
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/* Set ReadError on bios that experience a readerror so that
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* raid1d knows what to do with them.
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*/
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R1BIO_ReadError,
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/* For write-behind requests, we call bi_end_io when
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* the last non-write-behind device completes, providing
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* any write was successful. Otherwise we call when
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* any write-behind write succeeds, otherwise we call
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* with failure when last write completes (and all failed).
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* Record that bi_end_io was called with this flag...
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*/
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R1BIO_Returned,
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/* If a write for this request means we can clear some
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* known-bad-block records, we set this flag
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*/
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R1BIO_MadeGood,
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R1BIO_WriteError,
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R1BIO_FailFast,
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};
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static inline int sector_to_idx(sector_t sector)
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{
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return hash_long(sector >> BARRIER_UNIT_SECTOR_BITS,
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BARRIER_BUCKETS_NR_BITS);
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}
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#endif
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