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Linux AIO Performance and Robustness for Enterprise Workloads
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Suparna Bhattacharya
John Tran
Mike Sullivan
Chris Mason
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{suparna@in, jbtran@us, mksully@us}.ibm.com, mason@suse.com
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Overview of kernel AIO in Linux(tm) 2.6
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AIO overlaps processing with I/O operations
improved utilization of CPU and devices
especially under variable loads
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AIO syscalls
io_setup(max_events, &ctx)
io_submit(ctx, nr, iocbs[])
IO_CMD_PREAD, IO_CMD_PWRITE, IO_CMD_POLL
io_getevents(ctx, min_nr, nr, events[], timeout)
io_destroy(ctx)
io_cancel(ctx, iocb, &result)
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Retry-based AIO Recap
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Sync I/O and AIO share a common code path
Async io_wait context for AIO
Allocated as part of kiocb
Replaces default on-stack wait queue entry for sync I/O
Retry exit for AIO instead of blocking
Return number of bytes completed or -EIOCBRETRY
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AIO proceeds as a series of non-blocking iterations
Retry kicked via async wait queue callback on wakeup
Reissues fop->aio_read/write
Modified arguments representing the remaining I/O
Worker threads use caller's address space during retries
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Impact of Readahead on AIO Reads
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Current Ahead
----|-------|--------|----------------|-----
^start ^start+size
^ahead_start ^ahead_start+ahead_size
^preoffset
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Impact on Streaming Random AIO Reads
fop->aio_read(fd, o1, 16384) = -EIOCBRETRY
Readahead o1 to o1+64KB, pre = o1
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fop->aio_read(fd, o2, 16384) = -EIOCBRETRY
Readahead o2 to o2+8KB, pre = o2
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fop->aio_read(fd, o3, 16384) = -EIOCBRETRY
No readahead, Slow read 4KB
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fop->aio_read(fd, o1, 16384) = 16384
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fop->aio_read(fd, o2, 16384) = 8192
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fop->aio_read(fd, o3, 16384) = 4096
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Upfront Readahead for AIO
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Tells readahead logic about all pages in a request
Doesn't repeat readahead on retries
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I/O Pattern with Streaming AIO Reads
fop->aio_read(fd, o1, 16384) = -EIOCBRETRY
Readahead o1 to 64KB, pre = o1+12KB
fop->aio_read(fd, o2, 16384) = -EIOCBRETRY
Readahead o2 to 20KB, pre = o2+16KB
fop->aio_read(fd, o1, 16384) = 16384
fop->aio_read(fd, o2, 16384) = 16384
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Addressing Sendfile regression
Upfront only within max readahead pages
Restrict to AIO case
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Random AIO Read Throughput
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Filesystem: ext3 4KB blocksize, 1GB file
AIC7896 Ultra2 SCSI
4-way Pentium(tm) III 700MHz, 512MB
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AIO DIO vs Cached I/O Integrity
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DIO vs Buffered I/O races (2.4 & 2.6) - sct
Meta-data mods vs actual disk block instantiation
Atomicity of flush & DIO read - i_sem
Truncate vs DIO write/read - r/w i_alloc_sem
DIO fallback to buffered I/O for writes to sparse regions
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AIO DIO specific races (2.6)
AIO-DIO read/write vs truncate
hold i_alloc_sem till I/O completion
AIO-DIO file extends
block instantiation atomicity with i_size updates
force synchronous behaviour
AIO-DIO writes to sparse regions
request spanning allocated & sparse regions
force synchronous behaviour
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Concurrent Synchronized I/O
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Synchronized Writes (O_SYNC, fsync)
I/O committed to disk before request completes
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Concurrent DIO writes
i_sem serializes parallel DIO writes
could be released after blocks looked up and I/O started (helps streaming AIO-DIO writes)
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Concurrent O_SYNC buffered writes
Per-address space page lists (dirty & writeback)
i_sem held across traversal to writeback completion serializes parallel O_SYNC writes & difficult to retry-enable for AIO
races between sync and background writes
Radix-tree lookup of range to O_SYNC
avoids i_sem across I/O waits & AIO retry friendly
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Tagged Radix-tree based Writeback
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Lookup dirty or writeback pages in O(log64(n))
Adds tag bits for each slot to each radix-tree node
To search keep going down sub-trees under slots with tag set
Tagged gang-lookup for in-order searches in a range
Replaces per-address space page list traversals
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To synchronize writes to disk ...
Radix-tree walk and writeout dirty pages in range
Radix-tree walk and wait on writeback pages in range
Repeatable logic for wait on writeback for AIO O_SYNC
Issue all writeouts for the range during first iteration
Wait for writeback completion converted a retry exit
Retries don't re-dirty pages, fall through to wait on writeback step
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Random AIO Write Throughput
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Filesystem: ext3 4KB blocksize, 1GB file
AIC7896 Ultra2 SCSI
4-way PIII 700MHz, 512MB
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Retry Storms & Filtered Waitqueues
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Hashed wait queues & overloaded wakeups can interact with AIO retry-logic in tricky ways
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CPU1 CPU2
lock_page(px)
...
unlock_page(px)
lock_page(py)
wait_on_page_writeback_wq(px) ...
unlock_page(py)
wakes up px
triggering a retry
<-----------------------------------
lock_page(px) wait_on_page_writeback_wq(py)
...
unlock_page(py) ---- wakes up py --- causes retry ---->
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Filtered wakeups ensure specificity of the wakeup to the specific object and reason for wakeup, eliminating redundant wakeups, so we no longer have to worry about situations like the above.
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AIO and Database Workloads
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DB2(R) page cleaners
Flush dirty buffer pool pages to disk
Number and behaviour can be tuned according to demand
Frees agent processes to be dedicated to processing txns
AIO reduces no. of page cleaner processes
Helps OLTP workloads
Stream random synchronized AIO writes to preallocated blocks
Individual request size = Database page size (e.g. 8KB)
Keep disk queues maximally utilized and limit contention
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AIO reads for prefetching data
Expected to help decision support workloads
Streaming large random AIO reads
Individual request size = Database extent size (e.g. 256KB)
Need to tune readahead setting for device for buffered AIO
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AIO OLTP Performance - Raw Devices
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Update-intensive OLTP database workload
Derived from a TPC benchmark, but in no way comparable to any TPC results
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DB2 V8, Linux 2.6.1, 2-way AMD Opteron,
QLogic 2342 FC, 2 storage servers x 8 disk enclosures x 14 disks each
RAID-0 configuration, stripe size 256KB
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Configuration Relative Throughput
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1 page cleaner with AIO 133
55 page cleaners without AIO 122
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Configuration Page Cleaner Writes (%)
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1 page cleaner with AIO 100
55 page cleaners without AIO 70
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AIO OLTP Performance - Filesystems
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Update-intensive OLTP database workload
Derived from a TPC benchmark, but in no way comparable to any TPC results
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DB2 V8, Linux 2.6.0+mm1, 2-way AMD Opteron
QLogic 2310 FC, 4 disk enclosures, each with 2 disk
RAID-0 arrays, stripe size 256KB
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Configuration Relative Throughput
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5 page cleaners with AIO (buffered) 113.7
5 page cleaners without AIO 100
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Configuration Page Cleaner Writes (%)
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5 page cleaners with AIO (buffered) 100
5 page cleaners without AIO 37
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Combining AIO & Communications I/O
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Different API models
Communications IO uses epoll + O_NONBLOCK
Disk-based AIO uses native AIO API
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Combining into a single event loop
Extend epoll for notification of AIO completion
AIO poll
IO_CMD_POLL (fd, event mask)
retry based implementation
Native AIO support for communications I/O
e.g. AIO pipe (retry-based implementation)
read immediate added to retry-model
context switch reduction vs IO stalls
AIO on sockets not implemented yet
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Conclusions
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In retrospect ...
Real challenges are beyond sync->async conversion
AIO exposed I/O patterns less likely with sync I/O alone
AIO appeared to magnify some problems early
hashed waitqueues -> filtered wakeups
readahead window collapse with large random reads
Enhancements improved concurrency for sync I/O paths
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Room for future work
More benchmarking, analysis & optimizations
AIO fsync
More widely used AIO applications
Study to determine if network AIO is worth it
Enabling efficient POSIX AIO implementations
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Our Thanks to ...
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Andrew Morton <akpm@digeo.com>
Daniel McNeil <daniel@osdl.org>
Janet Morgan <janetmor@us.ibm.com>
Badari Pulavarthy <pbadari@us.ibm.com>
Stephen Tweedie <sct@redhat.com>
William Lee Irwin <wli@holomorphy.com>
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and several other people on the linux-aio and the linux-kernel mailing lists for various suggestions and fixes
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Downloads:
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AIO patches discussed here are available at www.kernel.org/pub/linux/kernel/people/suparna/aio
AIO web page: http://lse.sf.net/io/aio.html
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Disclaimers and Trademarks
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This work represents the view of the authors and
does not necessarily represent the view of IBM.
IBM and DB2 are registered trademarks
of International Business Machines Corporation in the
United States and/or other countries.
Linux is a registered trademark of Linus Torvalds.
Pentium is a trademark of Intel Corporation in the
United States, other countries or both
Other company, product, and service names may be
trademarks or service marks of others.
The benchmarks discussed in this presentation were
conducted for research purposes only, under laboratory
conditions. Results will not be realised in all
computing environments
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