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Asynchronous IO for Linux

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Benjamin LaHaise
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bcrl@redhat.com
{bcrl,blah,kernel}@kvack.org

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Overview
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	why do we need AIO?
	implementation
		userland API
		kernel API
		kernel extensions
	current status / todo

#			file operations
#			wait queue funcs
#			work to dos
#				page wait
#				page locking
#				buffer wait
#				buffer locking
#				semaphores
#			raw devices
#			generic page cache aio
#			networking
#			pipes
#		overhead
#
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Why AIO?
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	useful in event based applications
	required to make efficient use of raw io
	sematics fit with ''zero copy"
	significantly lower overhead for daemons vs select/poll
	scalable for large numbers of connections

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AIO Design Overview - User APIs
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	event driven: presents completed events to the user

	new device: /dev/aio
		context for io events
		handle for mmap

	new syscalls
		int __io_wait(int ctx_fd, int event_idx, void *key,
                           struct timespec *timeout);
		int __io_cancel(int ctx_fd, int event_idx, void *key);
		int __io_getevents(int ctx_fd, struct io_event *events,
                                 int nr, struct timespec *timeout);
		int __submit_ios(int ctx_fd, int nr, struct iocb **iocbs);

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AIO Design Overview - Kernel APIs
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	make use of existing asynchronous in kernel APIs where
     possible, fixing bugs as needed
		readpage/writepage
		submit_bh
	in-kernel API currently uses
            int rw_kiovec(struct file *filp, int rw, struct kiovec *iovec,
                               size_t size, loff_t pos);
	probable alternative
            int io_submit(struct file *filp, struct iocb *iocb);


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wait_queue_t extension
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	typedef void (*wait_queue_func_t)(wait_queue_t *wait)
	simple extension to wait queues:
		adds a wait_queue_func_t func; to wait_queue_t
		void init_waitqueue_func_entry(wait_queue_t *q,
                                 wait_queue_func_t func);
	provides a threadless means of receiving events in kernel
		lower overhead == better scalability
	works with existing wake all and exclusive semantics
	must be very careful about acquiring locks
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	Note!  Use of waitqueue_active introduces races!
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Work To Dos
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	based on ideas from Jeff Merkey
	context for state machine execution
	allows sharing of a pool of worker threads
	fully non blocking primatives
	based on task queues + wait queue

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Work To Dos
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	primatives for state driven code

		void wtd_set_action(struct worktodo *wtd, void (*f)(void *d), void *d);
		void wtd_queue(struct worktodo *wtd);
		void wtd_wait_page(struct worktodo *wtd, struct page *page);
		void wtd_lock_page(struct worktodo *wtd, struct page *page);
		void wtd_wait_on_buffer(struct worktodo *wtd, struct page *page);
		void wtd_lock_buffer(struct worktodo *wtd, struct page *page);
		void wtd_down(struct worktodo *wtd, struct page *page);

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Current Status
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	current patch is ~3000 line patch
	read/write implemented for:
		raw io
		generic page cache (write back and partial O_DSYNC)
			uses wtd operations
	tested and works well - nicely performant on RAID0 :-)

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ToDo
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	write documentation
	improve glibc code
	accounting
		limit kernel memory pinning
		outstanding io limits
		dirty buffer pressure
	socket send/receive
	integrate davem's single copy pipe code
	O_DIRECT support

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Current Status
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	Mailing list:
		linux-aio@kvack.org
		echo subscribe | mail -s foo majordomo@kvack.org

	Download:
		http://people.redhat.com/bcrl/
		http://www.kvack.org/~blah/