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![Ming Lei](/assets/img/avatar_default.png)
There are at least 3 advantages to use direct I/O and AIO on read/write loop's backing file: 1) double cache can be avoided, then memory usage gets decreased a lot 2) not like user space direct I/O, there isn't cost of pinning pages 3) avoid context switch for obtaining good throughput - in buffered file read, random I/O top throughput is often obtained only if they are submitted concurrently from lots of tasks; but for sequential I/O, most of times they can be hit from page cache, so concurrent submissions often introduce unnecessary context switch and can't improve throughput much. There was such discussion[1] to use non-blocking I/O to improve the problem for application. - with direct I/O and AIO, concurrent submissions can be avoided and random read throughput can't be affected meantime xfstests(-g auto, ext4) is basically passed when running with direct I/O(aio), one exception is generic/232, but it failed in loop buffered I/O(4.2-rc6-next-20150814) too. Follows the fio test result for performance purpose: 4 jobs fio test inside ext4 file system over loop block 1) How to run - KVM: 4 VCPUs, 2G RAM - linux kernel: 4.2-rc6-next-20150814(base) with the patchset - the loop block is over one image on SSD. - linux psync, 4 jobs, size 1500M, ext4 over loop block - test result: IOPS from fio output 2) Throughput(IOPS) becomes a bit better with direct I/O(aio) ------------------------------------------------------------- test cases |randread |read |randwrite |write | ------------------------------------------------------------- base |8015 |113811 |67442 |106978 ------------------------------------------------------------- base+loop aio |8136 |125040 |67811 |111376 ------------------------------------------------------------- - somehow, it should be caused by more page cache avaiable for application or one extra page copy is avoided in case of direct I/O 3) context switch - context switch decreased by ~50% with loop direct I/O(aio) compared with loop buffered I/O(4.2-rc6-next-20150814) 4) memory usage from /proc/meminfo ------------------------------------------------------------- | Buffers | Cached ------------------------------------------------------------- base | > 760MB | ~950MB ------------------------------------------------------------- base+loop direct I/O(aio) | < 5MB | ~1.6GB ------------------------------------------------------------- - so there are much more page caches available for application with direct I/O [1] https://lwn.net/Articles/612483/ Signed-off-by: Ming Lei <ming.lei@canonical.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
93 lines
2.3 KiB
C
93 lines
2.3 KiB
C
/*
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* loop.h
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*
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* Written by Theodore Ts'o, 3/29/93.
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*
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* Copyright 1993 by Theodore Ts'o. Redistribution of this file is
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* permitted under the GNU General Public License.
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*/
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#ifndef _LINUX_LOOP_H
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#define _LINUX_LOOP_H
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#include <linux/bio.h>
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#include <linux/blkdev.h>
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#include <linux/blk-mq.h>
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#include <linux/spinlock.h>
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#include <linux/mutex.h>
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#include <linux/kthread.h>
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#include <uapi/linux/loop.h>
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/* Possible states of device */
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enum {
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Lo_unbound,
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Lo_bound,
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Lo_rundown,
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};
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struct loop_func_table;
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struct loop_device {
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int lo_number;
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atomic_t lo_refcnt;
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loff_t lo_offset;
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loff_t lo_sizelimit;
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int lo_flags;
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int (*transfer)(struct loop_device *, int cmd,
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struct page *raw_page, unsigned raw_off,
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struct page *loop_page, unsigned loop_off,
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int size, sector_t real_block);
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char lo_file_name[LO_NAME_SIZE];
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char lo_crypt_name[LO_NAME_SIZE];
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char lo_encrypt_key[LO_KEY_SIZE];
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int lo_encrypt_key_size;
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struct loop_func_table *lo_encryption;
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__u32 lo_init[2];
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kuid_t lo_key_owner; /* Who set the key */
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int (*ioctl)(struct loop_device *, int cmd,
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unsigned long arg);
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struct file * lo_backing_file;
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struct block_device *lo_device;
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unsigned lo_blocksize;
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void *key_data;
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gfp_t old_gfp_mask;
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spinlock_t lo_lock;
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int lo_state;
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struct mutex lo_ctl_mutex;
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struct kthread_worker worker;
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struct task_struct *worker_task;
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bool use_dio;
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struct request_queue *lo_queue;
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struct blk_mq_tag_set tag_set;
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struct gendisk *lo_disk;
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};
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struct loop_cmd {
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struct kthread_work work;
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struct request *rq;
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struct list_head list;
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bool use_aio; /* use AIO interface to handle I/O */
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struct kiocb iocb;
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};
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/* Support for loadable transfer modules */
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struct loop_func_table {
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int number; /* filter type */
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int (*transfer)(struct loop_device *lo, int cmd,
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struct page *raw_page, unsigned raw_off,
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struct page *loop_page, unsigned loop_off,
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int size, sector_t real_block);
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int (*init)(struct loop_device *, const struct loop_info64 *);
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/* release is called from loop_unregister_transfer or clr_fd */
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int (*release)(struct loop_device *);
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int (*ioctl)(struct loop_device *, int cmd, unsigned long arg);
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struct module *owner;
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};
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int loop_register_transfer(struct loop_func_table *funcs);
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int loop_unregister_transfer(int number);
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#endif
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