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Date: Thu, 5 Jan 2012 15:42:43 -0800
From: Tejun Heo <tj@...nel.org>
To: axboe@...nel.dk, mingo@...hat.com, rostedt@...dmis.org,
fweisbec@...il.com, teravest@...gle.com, slavapestov@...gle.com,
ctalbott@...gle.com, dsharp@...gle.com
Cc: linux-kernel@...r.kernel.org
Subject: [RFC PATCHSET RESEND] ioblame: statistical IO analyzer
This is re-post. The original posting was on Dec 15th but it was
missing cc to LKML. I got some responses on that thread so didn't
suspect LKML was missing. If you're getting it the second time. My
apologies.
Stuff pointed out in the original thread are...
* Is the quick variant of backtrace gathering really necessary? -
Still need to get performance numbers.
* TRACE_EVENT_CONDITION() can be used in some places. - Will be
updated.
Original message follows. Thanks.
Hello, guys.
Even with blktrace and tracepoints, getting insight into the IOs going
on a system is very challenging. A lot of IO operations happen long
after the action which triggered the IO finished and the overall
asynchronous nature of IO operations make it difficult to trace back
the origin of a given IO.
ioblame is an attempt at providing better visibility into overall IO
behavior. ioblame hooks into various tracepoints and tries to
determine who caused any given IO how and charges the IO accordingly.
On each IO completion, ioblame knows who to charge the IO (task), how
the IO got triggered (stack trace at the point of triggering, be it
page, inode dirtying or direct IO issue) and various information about
the IO itself (offset, size, how long it took and so on). ioblame
collects this information into histograms which is configurable from
userland using debugfs interface.
For example, using ioblame, user can acquire information like "this
task triggered IO with this stack trace on this file with the
following offset distribution".
For more details, please read Documentation/trace/ioblame.txt, which
I'll append to this message too for discussion.
This patchset contains the following 11 patches.
0001-trace_event_filter-factorize-filter-creation.patch
0002-trace_event_filter-add-trace_event_filter_-interface.patch
0003-block-block_bio_complete-tracepoint-was-missing.patch
0004-block-add-req-to-bio_-front-back-_merge-tracepoints.patch
0005-block-abstract-disk-iteration-into-disk_iter.patch
0006-writeback-move-struct-wb_writeback_work-to-writeback.patch
0007-writeback-add-more-tracepoints.patch
0008-block-add-block_touch_buffer-tracepoint.patch
0009-vfs-add-fcheck-tracepoint.patch
0010-stacktrace-implement-save_stack_trace_quick.patch
0011-block-trace-implement-ioblame-IO-statistical-analyze.patch
0001-0002 export trace_event_filter so that ioblame can use it too.
0003 adds back block_bio_complete TP invocation, which got lost
somehow. This probably makes sense as fix patch for 3.2.
0004-0006 update block layer in preparation. 0005 probably makes
sense as a standalone patch too.
0007-0009 add more tracepoints along the IO stack.
0010 adds nimbler backtrace dump function as ioblame dumps stacktrace
extremely frequently.
0011 implements ioblame.
This is still in early stage and I haven't done much performance
analysis yet. Tentative testing shows it adds ~20% CPU overhead when
used on memory backed loopback device.
The patches are on top of mainline (42ebfc61cf "Merge branch
'stable...git/konrad/xen'") and perf/core (74eec26fac "perf tools: Add
ability to synthesize event according to a sample").
It's also available in the following git branch.
git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git review-ioblame
diffstat follows.
Documentation/trace/ioblame.txt | 646 ++++++
arch/x86/include/asm/stacktrace.h | 2
arch/x86/kernel/stacktrace.c | 40
block/blk-core.c | 5
block/genhd.c | 98 -
fs/bio.c | 3
fs/fs-writeback.c | 34
fs/super.c | 2
include/linux/blk_types.h | 7
include/linux/buffer_head.h | 7
include/linux/fdtable.h | 3
include/linux/fs.h | 3
include/linux/genhd.h | 14
include/linux/ioblame.h | 95 +
include/linux/stacktrace.h | 6
include/linux/writeback.h | 18
include/trace/events/block.h | 70
include/trace/events/vfs.h | 40
include/trace/events/writeback.h | 113 +
kernel/stacktrace.c | 6
kernel/trace/Kconfig | 11
kernel/trace/Makefile | 1
kernel/trace/blktrace.c | 2
kernel/trace/ioblame.c | 3479 +++++++++++++++++++++++++++++++++++++
kernel/trace/trace.h | 6
kernel/trace/trace_events_filter.c | 379 ++--
mm/page-writeback.c | 2
27 files changed, 4872 insertions(+), 220 deletions(-)
Thanks.
--
tejun
ioblame - statistical IO analyzer with origin tracking
December, 2011 Tejun Heo <tj@...nel.org>
CONTENTS
1. Introduction
2. Overall design
3. Debugfs interface
3-1. Configuration
3-2. Monitoring
3-3. Data acquisition
4. Notes
5. Overheads
1. Introduction
In many workloads, IO throughput and latency have large effect on
overall performance; however, due to the complexity and asynchronous
nature, it is very difficult to characterize what's going on.
blktrace and various tracepoints provide visibility into individual IO
operations but it is still extremely difficult to trace back to the
origin of those IO operations.
ioblame is statistical IO analyzer which can track and record origin
of IOs. It keeps track of who dirtied pages and inodes, and, on an
actual IO, attributes it to the originator of the IO.
The design goals of ioblame are
* Minimally invasive - Analyzer shouldn't be invasive. Except for
adding some fields to mostly block layer data structures for
tracking, ioblame gathers all information through well defined
tracepoints and all tracking logic is contained in ioblame proper.
* Generic and detailed - There are many different IO paths and
filesystems which also go through changes regularly. Analyzer
should be able to report detailed enough result covering most cases
without requiring frequent adaptation. ioblame uses stack trace at
key points combined information from generic layers to categorize
IOs. This gives detailed enough information into varying IO paths
without requiring specific adaptations.
* Low overhead - Overhead both in terms of memory and processor cycles
should be low enough so that the analyzer can be used in IO-heavy
production environments. ioblame keeps hot data structures compact
and mostly read-only and avoids synchronization on hot paths by
using RCU and taking advantage of the fact that statistics doesn't
have to be completely accurate.
* Dynamic and customizable - There are many different aspects of IOs
which can be irrelevant or interesting depending on the situation.
From analysis point of view, always recording all collected data
would be ideal but is very wasteful in most situations. ioblame
lets users decide what information to gather so that they can
acquire relevant information without wasting resources
unnecessarily. ioblame also allows dynamic configuration while the
analyzer is online which enables dynamic drill down of IO behaviors.
2. Overall design
ioblame tracks the following three object types.
* Role: This tracks 'who' is taking an action. Normally corresponds
to a thread. Can also be specified by userland (not implemented
yet).
* Intent: Stack trace + modifier. An intent groups actions of the
same type. As the name suggests, modifier modifies the intent and
there can be multiple intents with the same stack trace but
different modifiers. Currently, only writeback modifiers are
implemented which denote why the writeback action is occurring -
ie. wb_reason.
* Act: This is combination of role, intent and the inode being
operated on and the ultimate tracking unit ioblame uses. IOs are
charged to and statistics are gathered by acts.
ioblame uses the same indexing data structure for all three types of
objects. Objects are never linked directly using pointers and every
access goes through the index. This allows avoiding expensive strict
object lifetime management. Objects are located either by its content
via hash table or id which contains generation number.
To attribute data writebacks to the originator, ioblame maintains a
table indexed by page frame number which keeps track of which act
dirtied which pages. For each IO, the target pages are looked up in
the table and the dirtying act is charged for the IO. Note that,
currently, each IO is charged as whole to a single act - e.g. all of
an IO for writeback encompassing multiple dirtiers will be charged to
the first found dirtying act. This simplifies data collection and
reporting while not losing too much information - writebacks tend to
be naturally grouped and IOPS (IO operations per second) are often
more significant than length of each IO.
inode writeback tracking is more involved as different filesystems
handle metadata updates and writebacks differently. ioblame uses
per-inode and buffer_head operation tracking to identify inode
writebacks to the originator.
After all the tracking, on each IO completion, ioblame knows the
offset and size of the IO, the act to be charged, how long it took in
the queue and device. From the information, ioblame produces fields
which can be recorded.
All statistics are recorded in histograms, called counters, which have
eight slots. Userland can specify the number of counters, IO
directions to consider, what each counter records, the boundary values
to decide histogram slots and optional filter for more complex
filtering conditions.
All interactions including configuration and data acquisition happen
via files under /sys/kernel/debug/ioblame/.
3. Debugfs interface
3-1. Configuration
* devs - can be changed anytime
Specifies the devices ioblame is enabled for. ioblame will only
track operations on devices which are explicitly enabled in this
file.
It accepts white space separated list of MAJ:MINs or block device
names with optional preceding '!' for negation. Opening with
O_TRUNC clears all existing entries. For example,
$ echo sda sdb > devs # disables all devices and then enable sd[ab]
$ echo sdc >> devs # sd[abc] enabled
$ echo !8:0 >> devs # sd[bc] enabled
$ cat devs
8:16 sdb
8:32 sdc
* max_{role|intent|act}s - can be changed while disabled
Specifies the maximum number of each object type. If the number of
certain object type exceeds the limit, IOs will be attributed to
special NOMEM object.
* ttl_secs - can be changed anytime
Specifies TTL of roles and acts. Roles are reclaimed after at least
TTL has passed after the matching thread has exited or execed and
assumed another tid. Acts are reclaimed after being unused for at
least TTL.
Note that reclaiming is tied to userland reading counters data. If
userland doesn't read counters, reclaiming won't happen.
* nr_counters - can be changed while disabled
Specifies the number of counters. Each act will have the specified
number of histograms associated with it. Individual counters can be
configured using files under the counters subdirectory. Any write
to this file clears all counter settings.
* counters/NR - can be changed anytime
Specifies each counter. Its format is
DIR FIELD B0 B1 B2 B3 B4 B5 B6 B7 B8
DIR is any combination of letters 'R', 'A', and 'W', each
representing reads (sans readaheads), readaheads and writes.
FIELD is the field to record in histogram and one of the followings.
offset : IO offset scaled to 0-65535
size : IO size
wait_time : time spent queued in usecs
io_time : time spent on device in usecs
seek_dist : seek dist from IO completed right before, scaled to 0-65536
B[0-8] are the boundaries for the histogram. Histograms have eight
slots. If (FIELD < B[0] || (B[8] != 0 && FIELD >= B[8])), it's not
recorded; otherwise, FIELD is counted in the slot with enclosing
boundaries. e.g. If FIELD is >= B2 and < B3, it's recorded in the
second slot (slot 1).
B8 can be zero indicating no upper limit but all other boundaries
must be equal to or larger than the boundary before them.
e.g. To record offsets of reads and read aheads in counter 0,
$ echo RA offset 0 8192 16384 24576 32768 40960 49152 57344 0 > counters/0
If higher resolution than 8 slots is necessary, two counters can be
used.
$ echo RA offset 0 4096 8192 12288 16384 20480 24576 28672 32768 > counters/0
$ echo RA offset 32768 36864 40960 45056 49152 53248 57344 61440 0 \
> counters/1
Writing empty string disables the counter.
$ echo > 1
$ cat 1
--- disabled
* counters/NR_filter - can be changed anytime
Specifies trace event type filter for more complex conditions. For
example, it allows conditions like "if IO is in the latter half of
the device, size is smaller than 128k and IO time is equal to or
longer than 10ms".
To record IO time in counter 0 with the above condition,
$ echo 'offset >= 16384 && size < 131072 && io_time >= 10000' > 0_filter
$ echo RAW io_time 10000 25000 50000 100000 500000 1000000 2500000 \
5000000 0 > 0
Any FIELD can be used in filter specification. For more details
about filter format, please read "Event filtering" section in
Documentation/trace/events.txt.
Writing '0' to filter file removes the filter. Note that writing
malformed filter disables the filter and reading it back afterwards
returns error message explaining why parsing failed.
3-2. Monitoring (read only)
* nr_{roles|intents|acts}
Returns the number of objects of the type. The number of roles and
acts can decrease after reclaiming but nr_intents only increases
while ioblame is enabled.
* stats/idx_nomem
How many times role, intent or act creation failed because memory
allocation failed while extending index to accomodate new object.
* stats/idx_nospc
How many times role, intent or act creation failed because limit
specified by {role|intent|act}_max is reached.
* stats/node_nomem
How many times role, intent or act creation failed to allocate.
* stats/pgtree_nomem
How many times page tree, which maps page frame number to dirtying
act, failed to expand due to memory allocation failure.
* stats/cnts_nomem
How many times per-act counter allocation failed.
* stats/iolog_overflow
How many iolog entries are lost due to overflow.
3-3. Data acquisition (read only)
* iolog
iolog is primarily a debug feature and dumps IOs as they complete.
$ cat iolog
R 4096 @ 74208 pid-5806 (ls) dev=0x800010 ino=0x2 gen=0x0
#39 modifier=0x0
[ffffffff811a0696] submit_bh+0xe6/0x120
[ffffffff811a1f56] ll_rw_block+0xa6/0xb0
[ffffffff81239a43] ext4_bread+0x43/0x80
[ffffffff8123f4e3] htree_dirblock_to_tree+0x33/0x190
[ffffffff8123f79a] ext4_htree_fill_tree+0x15a/0x250
[ffffffff8122e26e] ext4_readdir+0x10e/0x5d0
[ffffffff811832d0] vfs_readdir+0xa0/0xc0
[ffffffff81183450] sys_getdents+0x80/0xe0
[ffffffff81a3c8bb] system_call_fastpath+0x16/0x1b
W 4096 @ 0 pid-20 (sync_supers) dev=0x800010 ino=0x0 gen=0x0
#44 modifier=0x0
[ffffffff811a0696] submit_bh+0xe6/0x120
[ffffffff811a371d] __sync_dirty_buffer+0x4d/0xd0
[ffffffff811a37ae] sync_dirty_buffer+0xe/0x10
[ffffffff81250ee8] ext4_commit_super+0x188/0x230
[ffffffff81250fae] ext4_write_super+0x1e/0x30
[ffffffff811738fa] sync_supers+0xfa/0x100
[ffffffff8113d3e1] bdi_sync_supers+0x41/0x60
[ffffffff810ad4c6] kthread+0x96/0xa0
[ffffffff81a3dcb4] kernel_thread_helper+0x4/0x10
W 4096 @ 8512 pid-5813 dev=0x800010 ino=0x74 gen=0x4cc12c59
#45 modifier=0x10000002
[ffffffff812342cb] ext4_setattr+0x25b/0x4c0
[ffffffff8118b9ba] notify_change+0x10a/0x2b0
[ffffffff8119ef87] utimes_common+0xd7/0x180
[ffffffff8119f0c9] do_utimes+0x99/0xf0
[ffffffff8119f21d] sys_utimensat+0x2d/0x90
[ffffffff81a3c8bb] system_call_fastpath+0x16/0x1b
...
The first entry is 4k read at sector 74208 (unscaled) on /dev/sdb
issued by ls. The second is sync_supers writing out dirty super
block. The third is inode writeback from "touch FILE; sync". Note
that the modifier is set (it's indicating WB_REASON_SYNC).
Here is another example from "cp FILE FILE1" and then waiting.
W 4096 @ 0 pid-20 (sync_supers) dev=0x800010 ino=0x0 gen=0x0
#16 modifier=0x0
[ffffffff8139cd94] submit_bio+0x74/0x100
[ffffffff811cba3b] submit_bh+0xeb/0x130
[ffffffff811cecd2] __sync_dirty_buffer+0x52/0xd0
[ffffffff811ced63] sync_dirty_buffer+0x13/0x20
[ffffffff81281fa8] ext4_commit_super+0x188/0x230
[ffffffff81282073] ext4_write_super+0x23/0x40
[ffffffff8119c8d2] sync_supers+0x102/0x110
[ffffffff81162c99] bdi_sync_supers+0x49/0x60
[ffffffff810bc216] kthread+0xb6/0xc0
[ffffffff81ab13b4] kernel_thread_helper+0x4/0x10
...
W 4096 @ 8512 pid-668 dev=0x800010 ino=0x73 gen=0x17b5119d
#23 modifier=0x10000003
[ffffffff811c55b0] __mark_inode_dirty+0x220/0x330
[ffffffff811cccfb] generic_write_end+0x6b/0xa0
[ffffffff81268b10] ext4_da_write_end+0x150/0x360
[ffffffff811444bb] generic_file_buffered_write+0x18b/0x290
[ffffffff81146938] __generic_file_aio_write+0x238/0x460
[ffffffff81146bd8] generic_file_aio_write+0x78/0xf0
[ffffffff8125ef9f] ext4_file_write+0x6f/0x2a0
[ffffffff811997f2] do_sync_write+0xe2/0x120
[ffffffff8119a428] vfs_write+0xc8/0x180
[ffffffff8119a5e1] sys_write+0x51/0x90
[ffffffff81aafe2b] system_call_fastpath+0x16/0x1b
...
W 524288 @ 3276800 pid-668 dev=0x800010 ino=0x73 gen=0x17b5119d
#25 modifier=0x10000003
[ffffffff811cc86c] __set_page_dirty+0x4c/0xd0
[ffffffff811cc956] mark_buffer_dirty+0x66/0xa0
[ffffffff811cca39] __block_commit_write+0xa9/0xe0
[ffffffff811ccc42] block_write_end+0x42/0x90
[ffffffff811cccc3] generic_write_end+0x33/0xa0
[ffffffff81268b10] ext4_da_write_end+0x150/0x360
[ffffffff811444bb] generic_file_buffered_write+0x18b/0x290
[ffffffff81146938] __generic_file_aio_write+0x238/0x460
[ffffffff81146bd8] generic_file_aio_write+0x78/0xf0
[ffffffff8125ef9f] ext4_file_write+0x6f/0x2a0
[ffffffff811997f2] do_sync_write+0xe2/0x120
[ffffffff8119a428] vfs_write+0xc8/0x180
[ffffffff8119a5e1] sys_write+0x51/0x90
[ffffffff81aafe2b] system_call_fastpath+0x16/0x1b
...
The first entry is ext4 marking super block dirty. After a while,
periodic writeback kicks in (modifier 0x100000003) and the inode
dirtied by cp is written back followed by dirty data pages.
At this point, iolog is mostly a debug feature. The output format
is quite verbose and it isn't particularly performant. If
necessary, it can be extended to use trace ringbuffer and grow
per-cpu binary interface.
* intents
Dump of intents in Human readable form.
$ cat intents
#0 modifier=0x0
#1 modifier=0x0
#2 modifier=0x0
[ffffffff81189a6a] file_update_time+0xca/0x150
[ffffffff81122030] __generic_file_aio_write+0x200/0x460
[ffffffff81122301] generic_file_aio_write+0x71/0xe0
[ffffffff8122ea94] ext4_file_write+0x64/0x280
[ffffffff811b5d24] aio_rw_vect_retry+0x74/0x1d0
[ffffffff811b7401] aio_run_iocb+0x61/0x190
[ffffffff811b81c8] do_io_submit+0x648/0xaf0
[ffffffff811b867b] sys_io_submit+0xb/0x10
[ffffffff81a3c8bb] system_call_fastpath+0x16/0x1b
#3 modifier=0x0
[ffffffff811aaf2e] __blockdev_direct_IO+0x1f1e/0x37c0
[ffffffff812353b2] ext4_direct_IO+0x1b2/0x3f0
[ffffffff81121d6a] generic_file_direct_write+0xba/0x180
[ffffffff8112210b] __generic_file_aio_write+0x2db/0x460
[ffffffff81122301] generic_file_aio_write+0x71/0xe0
[ffffffff8122ea94] ext4_file_write+0x64/0x280
[ffffffff811b5d24] aio_rw_vect_retry+0x74/0x1d0
[ffffffff811b7401] aio_run_iocb+0x61/0x190
[ffffffff811b81c8] do_io_submit+0x648/0xaf0
[ffffffff811b867b] sys_io_submit+0xb/0x10
[ffffffff81a3c8bb] system_call_fastpath+0x16/0x1b
#4 modifier=0x0
[ffffffff811aaf2e] __blockdev_direct_IO+0x1f1e/0x37c0
[ffffffff8126da71] ext4_ind_direct_IO+0x121/0x460
[ffffffff81235436] ext4_direct_IO+0x236/0x3f0
[ffffffff81122db2] generic_file_aio_read+0x6b2/0x740
...
The # prefixed number is the NR of the intent used to link intent
from stastics. Modifier and stack trace follow. The first two
entries are special - 0 is nomem intent and 1 is lost intent. The
former is used when an intent can't be created because allocation
failed or intent_max is reached. The latter is used when reclaiming
resulted in loss of tracking info and the IO can't be reported
exactly.
This file can be seeked by intent NR. ie. seeking to 3 and reading
will return intent #3 and after. Because intents are never
destroyed while ioblame is enabled, this allows userland tool to
discover new intents since last reading. Seeking to the number of
currently known intents and reading returns only the newly created
intents.
* intents_bin
Identical to intents but in compact binary format and likely to be
much more performant. Each entry in the file is in the following
format as defined in include/linux/ioblame.h.
#define IOB_INTENTS_BIN_VER 1
/* intent modifer */
#define IOB_MODIFIER_TYPE_SHIFT 28
#define IOB_MODIFIER_TYPE_MASK 0xf0000000U
#define IOB_MODIFIER_VAL_MASK (~IOB_MODIFIER_TYPE_MASK)
/* val contains wb_reason */
#define IOB_MODIFIER_WB (1 << IOB_MODIFIER_TYPE_SHIFT)
/* record struct for /sys/kernel/debug/ioblame/intents_bin */
struct iob_intent_bin_record {
uint16_t len;
uint16_t ver;
uint32_t nr;
uint32_t modifier;
uint32_t __pad;
uint64_t trace[];
} __packed;
* counters_pipe
Dumps counters and triggers reclaim. Opening and reading this file
returns counters of all acts which have been used since the last
open.
Because roles and acts shouldn't be reclaimed before the updated
counters are reported, reclaiming is tied to counters_pipe access.
Opening counters_pipe prepares for reclaiming and closing executes
it. Act reclaiming works at ttl_secs / 2 granularity. ioblame
tries to stay close to the lifetime timings requested by ttl_secs
but note that reclaim happens only on counters_pipe open/close.
There can only be one user of counters_pipe at any given moment;
otherwise, file operations will fail and the output and reclaiming
timings are undefined.
All reported histogram counters are u32 and never reset. It's the
user's responsibility to calculate the delta if necessary. Note
that counters_pipe reports all used acts since last open and
counters are not guaranteed to have been updated - ie. there can be
spurious acts in the output.
counters_pipe is seekable by act NR.
In the following example, two counters are configured - the first
one counts read offsets and the second write offsets. A file is
copied using dd w/ direct flags.
$ cat counters_pipe
pid-20 (sync_supers) int=66 dev=0x800010 ino=0x0 gen=0x0
0 0 0 0 0 0 0 0
2 0 0 0 0 0 0 0
pid-1708 int=58 dev=0x800010 ino=0x71 gen=0x3e0d99f2
11 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
pid-1708 int=59 dev=0x800010 ino=0x71 gen=0x3e0d99f2
11 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
pid-1708 int=62 dev=0x800010 ino=0x2727 gen=0xf4739822
0 0 0 0 0 0 0 0
10 0 0 0 0 0 0 0
pid-1708 int=63 dev=0x800010 ino=0x2727 gen=0xf4739822
0 0 0 0 0 0 0 0
10 0 0 0 0 0 0 0
pid-1708 int=31 dev=0x800010 ino=0x2727 gen=0xf4739822
0 0 0 0 0 0 0 0
2 0 0 0 0 0 0 0
pid-1708 int=65 dev=0x800010 ino=0x2727 gen=0xf4739822
0 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
pid-1708 is the dd which copied the file. The output is separated
by pid-* lines and each section corresponds to single act, which has
intent NR and file (dev:ino:gen) associated with it. One 8-slot
histogram is printed per line in ascending order.
The filesystem is mostly empty and, from the output, both files seem
to be located in the first 1/8th of the disk.
All reads happened through intent 58 and 59. From intents file,
they are,
#58 modifier=0x0
[ffffffff8139d974] submit_bio+0x74/0x100
[ffffffff811d5dba] __blockdev_direct_IO+0xc2a/0x3830
[ffffffff8129fe51] ext4_ind_direct_IO+0x121/0x470
[ffffffff8126678e] ext4_direct_IO+0x23e/0x400
[ffffffff81147b05] generic_file_aio_read+0x6d5/0x760
[ffffffff81199932] do_sync_read+0xe2/0x120
[ffffffff8119a5c5] vfs_read+0xc5/0x180
[ffffffff8119a781] sys_read+0x51/0x90
[ffffffff81ab1fab] system_call_fastpath+0x16/0x1b
#59 modifier=0x0
[ffffffff8139d974] submit_bio+0x74/0x100
[ffffffff811d7345] __blockdev_direct_IO+0x21b5/0x3830
[ffffffff8129fe51] ext4_ind_direct_IO+0x121/0x470
[ffffffff8126678e] ext4_direct_IO+0x23e/0x400
[ffffffff81147b05] generic_file_aio_read+0x6d5/0x760
[ffffffff81199932] do_sync_read+0xe2/0x120
[ffffffff8119a5c5] vfs_read+0xc5/0x180
[ffffffff8119a781] sys_read+0x51/0x90
[ffffffff81ab1fab] system_call_fastpath+0x16/0x1b
Except for hitting slightly different paths in __blockdev_direct_IO,
they both are ext4 direct reads as expected. Writes seem more
diversified and upon examination, #62 and #63 are ext4 direct
writes. #31 and #65 are more interesting.
#31 modifier=0x0
[ffffffff811cd0cc] __set_page_dirty+0x4c/0xd0
[ffffffff811cd1b6] mark_buffer_dirty+0x66/0xa0
[ffffffff811cd299] __block_commit_write+0xa9/0xe0
[ffffffff811cd4a2] block_write_end+0x42/0x90
[ffffffff811cd523] generic_write_end+0x33/0xa0
[ffffffff81269720] ext4_da_write_end+0x150/0x360
[ffffffff81144878] generic_file_buffered_write+0x188/0x2b0
[ffffffff81146d18] __generic_file_aio_write+0x238/0x460
[ffffffff81146fb8] generic_file_aio_write+0x78/0xf0
[ffffffff8125fbaf] ext4_file_write+0x6f/0x2a0
[ffffffff81199812] do_sync_write+0xe2/0x120
[ffffffff8119a308] vfs_write+0xc8/0x180
[ffffffff8119a4c1] sys_write+0x51/0x90
[ffffffff81ab1fab] system_call_fastpath+0x16/0x1b
This is buffered write. It turns out the file size didn't match the
specified blocksize, so dd turns off O_DIRECT for the last IO and
issue buffered write for the remainder.
Note that the actual IO submission is not visible in the stack trace
as the IOs are charged to the dirtying act. Actual IOs are likely
to be executed from fsync(2).
#65 modifier=0x0
[ffffffff811c5e10] __mark_inode_dirty+0x220/0x330
[ffffffff81267edd] ext4_da_update_reserve_space+0x13d/0x230
[ffffffff8129006d] ext4_ext_map_blocks+0x13dd/0x1dc0
[ffffffff81268a31] ext4_map_blocks+0x1b1/0x260
[ffffffff81269c52] mpage_da_map_and_submit+0xb2/0x480
[ffffffff8126a84a] ext4_da_writepages+0x30a/0x6e0
[ffffffff8114f584] do_writepages+0x24/0x40
[ffffffff811468cb] __filemap_fdatawrite_range+0x5b/0x60
[ffffffff8114692a] filemap_write_and_wait_range+0x5a/0x80
[ffffffff8125ff64] ext4_sync_file+0x74/0x440
[ffffffff811ca31b] vfs_fsync_range+0x2b/0x40
[ffffffff811ca34c] vfs_fsync+0x1c/0x20
[ffffffff811ca58a] do_fsync+0x3a/0x60
[ffffffff811ca5e0] sys_fsync+0x10/0x20
[ffffffff81ab1fab] system_call_fastpath+0x16/0x1b
And this is dd fsync(2)ing and marking inode for writeback. As with
data writeback, IO submission is not visible in the stack trace.
* counters_pipe_bin
Identical to counters_pipe but in compact binary format and likely
to be much more performant. Each entry in the file is in the
following format as defined in include/linux/ioblame.h.
#define IOBC_PIPE_BIN_VER 1
/* record struct for /sys/kernel/debug/ioblame/counters_pipe_bin */
struct iobc_pipe_bin_record {
uint16_t len;
uint16_t ver;
int32_t id; /* >0 pid or negated user id */
uint32_t intent_nr; /* associated intent */
uint32_t dev;
uint64_t ino;
uint32_t gen;
uint32_t __pad;
uint32_t cnts[]; /* [type][slot] */
} __packed;
Note that counters_pipe and counters_pipe_bin can't be used
parallelly. Only one opener is allowed across the two files at any
given moment.
4. Notes
* By the time ioblame reports IOs or counters, the task which gets
charged might have already exited and this is why ioblame prints
task command in some reports but not in others. Userland tool is
advised to use combination of live task listing and process
accounting to match pid's to commands.
* dev:ino:gen can be mapped to filename without scanning the whole
filesystem by constructing FS-specific filehandle, opening it with
open_by_handle_at(2) and then readlink(2)ing /proc/self/FD. This
will return full path as long as the dentry is in cache, which is
likely if data acquisition and mapping don't happen too long after
IOs.
* Mechanism to specify userland role ID is not implemented yet.
5. Overheads
On x86_64, role is 104 bytes, intent 32 + 8 * stack_depth and act 72
bytes. Intents are allocated using kzalloc() and there shouldn't be
too many of them. Both roles and acts have their own kmem_cache and
can be monitored via /proc/slabinfo.
Each counter occupy 32 * nr_counters and is aligned to cacheline.
Counters are allocated only as necessary. iob_counters kmem_cache is
dynamically created on enable.
The size of page frame number -> dirtier mapping table is proportional
to the amount of available physical memory. If max_acts <= 65536,
2bytes are used per PAGE_SIZE. With 4k page, at most ~0.049% can be
used. If max_acts > 65536, 4bytes are used doubling the percentage to
~0.098%. The table also grows dynamically.
There are also indexing data structures used - hash tables, id[ra]s
and a radix tree. There are three hash tables, each sized according
to max_{roles|intents|acts}. The maximum memory usage by hash tables
is sizeof(void *) * (max_roles + max_intents + max_acts). Memory used
by other indexing structures should be negligible.
Preliminary tests w/ fio ssd-test on loopback device on tmpfs, which
is purely CPU cycle bound, shows ~20% throughput hit.
*** TODO: add performance testing results and explain involved CPU
overheads.
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