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Subject[patch v4 2/2] aio: implement request batching
Date
Hi,

Some workloads issue batches of small I/O, and the performance is poor
due to the call to blk_run_address_space for every single iocb. Nathan
Roberts pointed this out, and suggested that by deferring this call
until all I/Os in the iocb array are submitted to the block layer, we
can realize some impressive performance gains (up to 30% for sequential
4k reads in batches of 16).

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

diff --git a/fs/aio.c b/fs/aio.c
index 02a2c93..cf0bef4 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -32,6 +32,9 @@
#include <linux/workqueue.h>
#include <linux/security.h>
#include <linux/eventfd.h>
+#include <linux/blkdev.h>
+#include <linux/mempool.h>
+#include <linux/hash.h>

#include <asm/kmap_types.h>
#include <asm/uaccess.h>
@@ -60,6 +63,14 @@ static DECLARE_WORK(fput_work, aio_fput_routine);
static DEFINE_SPINLOCK(fput_lock);
static LIST_HEAD(fput_head);

+#define AIO_BATCH_HASH_BITS 3 /* allocated on-stack, so don't go crazy */
+#define AIO_BATCH_HASH_SIZE (1 << AIO_BATCH_HASH_BITS)
+struct aio_batch_entry {
+ struct hlist_node list;
+ struct address_space *mapping;
+};
+mempool_t *abe_pool;
+
static void aio_kick_handler(struct work_struct *);
static void aio_queue_work(struct kioctx *);

@@ -73,6 +84,8 @@ static int __init aio_setup(void)
kioctx_cachep = KMEM_CACHE(kioctx,SLAB_HWCACHE_ALIGN|SLAB_PANIC);

aio_wq = create_workqueue("aio");
+ abe_pool = mempool_create_kmalloc_pool(1, sizeof(struct aio_batch_entry));
+ BUG_ON(!abe_pool);

pr_debug("aio_setup: sizeof(struct page) = %d\n", (int)sizeof(struct page));

@@ -1531,8 +1544,44 @@ static int aio_wake_function(wait_queue_t *wait, unsigned mode,
return 1;
}

+static void aio_batch_add(struct address_space *mapping,
+ struct hlist_head *batch_hash)
+{
+ struct aio_batch_entry *abe;
+ struct hlist_node *pos;
+ unsigned bucket;
+
+ bucket = hash_ptr(mapping, AIO_BATCH_HASH_BITS);
+ hlist_for_each_entry(abe, pos, &batch_hash[bucket], list) {
+ if (abe->mapping == mapping)
+ return;
+ }
+
+ abe = mempool_alloc(abe_pool, GFP_KERNEL);
+ BUG_ON(!igrab(mapping->host));
+ abe->mapping = mapping;
+ hlist_add_head(&abe->list, &batch_hash[bucket]);
+ return;
+}
+
+static void aio_batch_free(struct hlist_head *batch_hash)
+{
+ struct aio_batch_entry *abe;
+ struct hlist_node *pos, *n;
+ int i;
+
+ for (i = 0; i < AIO_BATCH_HASH_SIZE; i++) {
+ hlist_for_each_entry_safe(abe, pos, n, &batch_hash[i], list) {
+ blk_run_address_space(abe->mapping);
+ iput(abe->mapping->host);
+ hlist_del(&abe->list);
+ mempool_free(abe, abe_pool);
+ }
+ }
+}
+
static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
- struct iocb *iocb)
+ struct iocb *iocb, struct hlist_head *batch_hash)
{
struct kiocb *req;
struct file *file;
@@ -1608,6 +1657,12 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
;
}
spin_unlock_irq(&ctx->ctx_lock);
+ if (req->ki_opcode == IOCB_CMD_PREAD ||
+ req->ki_opcode == IOCB_CMD_PREADV ||
+ req->ki_opcode == IOCB_CMD_PWRITE ||
+ req->ki_opcode == IOCB_CMD_PWRITEV)
+ aio_batch_add(file->f_mapping, batch_hash);
+
aio_put_req(req); /* drop extra ref to req */
return 0;

@@ -1635,6 +1690,7 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
struct kioctx *ctx;
long ret = 0;
int i;
+ struct hlist_head batch_hash[AIO_BATCH_HASH_SIZE] = { { 0, }, };

if (unlikely(nr < 0))
return -EINVAL;
@@ -1666,10 +1722,11 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
break;
}

- ret = io_submit_one(ctx, user_iocb, &tmp);
+ ret = io_submit_one(ctx, user_iocb, &tmp, batch_hash);
if (ret)
break;
}
+ aio_batch_free(batch_hash);

put_ioctx(ctx);
return i ? i : ret;
diff --git a/fs/direct-io.c b/fs/direct-io.c
index c86d35f..3af761c 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -1028,9 +1028,6 @@ direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode,
if (dio->bio)
dio_bio_submit(dio);

- /* All IO is now issued, send it on its way */
- blk_run_address_space(inode->i_mapping);
-
/*
* It is possible that, we return short IO due to end of file.
* In that case, we need to release all the pages we got hold on.
@@ -1057,8 +1054,11 @@ direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode,
((rw & READ) || (dio->result == dio->size)))
ret = -EIOCBQUEUED;

- if (ret != -EIOCBQUEUED)
+ if (ret != -EIOCBQUEUED) {
+ /* All IO is now issued, send it on its way */
+ blk_run_address_space(inode->i_mapping);
dio_await_completion(dio);
+ }

/*
* Sync will always be dropping the final ref and completing the

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