[PATCH 00/11] [RFC] 512K readahead size with thrashing safe readahead
From: Wu Fengguang
Date: Tue Feb 02 2010 - 10:36:43 EST
Andrew,
This is to lift default readahead size to 512KB, which I believe yields
more I/O throughput without noticeably increasing I/O latency for today's HDD.
For example, for a 100MB/s and 8ms access time HDD:
io_size KB access_time transfer_time io_latency util% throughput KB/s IOPS
4 8 0.04 8.04 0.49% 497.57 124.39
8 8 0.08 8.08 0.97% 990.33 123.79
16 8 0.16 8.16 1.92% 1961.69 122.61
32 8 0.31 8.31 3.76% 3849.62 120.30
64 8 0.62 8.62 7.25% 7420.29 115.94
128 8 1.25 9.25 13.51% 13837.84 108.11
256 8 2.50 10.50 23.81% 24380.95 95.24
512 8 5.00 13.00 38.46% 39384.62 76.92
1024 8 10.00 18.00 55.56% 56888.89 55.56
2048 8 20.00 28.00 71.43% 73142.86 35.71
4096 8 40.00 48.00 83.33% 85333.33 20.83
The 128KB => 512KB readahead size boosts IO throughput from ~13MB/s to ~39MB/s, while
merely increases IO latency from 9.25ms to 13.00ms.
As for SSD, I find that Intel X25-M SSD desires large readahead size
even for sequential reads (the first patch has benchmark details):
rasize first run time/throughput second run time/throughput
------------------------------------------------------------------
4k 3.40038 s, 123 MB/s 3.42842 s, 122 MB/s
8k 2.7362 s, 153 MB/s 2.74528 s, 153 MB/s
16k 2.59808 s, 161 MB/s 2.58728 s, 162 MB/s
32k 2.50488 s, 167 MB/s 2.49138 s, 168 MB/s
64k 2.12861 s, 197 MB/s 2.13055 s, 197 MB/s
128k 1.92905 s, 217 MB/s 1.93176 s, 217 MB/s
256k 1.75896 s, 238 MB/s 1.78963 s, 234 MB/s
512k 1.67357 s, 251 MB/s 1.69112 s, 248 MB/s
1M 1.62115 s, 259 MB/s 1.63206 s, 257 MB/s
2M 1.56204 s, 269 MB/s 1.58854 s, 264 MB/s
4M 1.57949 s, 266 MB/s 1.57426 s, 266 MB/s
As suggested by Linus, decrease default readahead size for small devices at the same time.
[PATCH 01/11] readahead: limit readahead size for small devices
[PATCH 02/11] readahead: bump up the default readahead size
[PATCH 03/11] readahead: introduce {MAX|MIN}_READAHEAD_PAGES macros for ease of use
The two other impacts of an enlarged readahead size are
- memory footprint (caused by readahead miss)
Sequential readahead hit ratio is pretty high regardless of max
readahead size; the extra memory footprint is mainly caused by
enlarged mmap read-around.
I measured my desktop:
- under Xwindow:
128KB readahead cache hit ratio = 143MB/230MB = 62%
512KB readahead cache hit ratio = 138MB/248MB = 55%
- under console: (seems more stable than the Xwindow data)
128KB readahead cache hit ratio = 30MB/56MB = 53%
1MB readahead cache hit ratio = 30MB/59MB = 51%
So the impact to memory footprint looks acceptable.
- readahead thrashing
It will now cost 1MB readahead buffer per stream. Memory tight systems
typically do not run multiple streams; but if they do so, it should
help I/O performance as long as we can avoid thrashing, which can be
achieved with the following patches.
[PATCH 04/11] readahead: replace ra->mmap_miss with ra->ra_flags
[PATCH 05/11] readahead: retain inactive lru pages to be accessed soon
[PATCH 06/11] readahead: thrashing safe context readahead
This is a major rewrite of the readahead algorithm, so I did careful tests with
the following tracing/stats patches:
[PATCH 07/11] readahead: record readahead patterns
[PATCH 08/11] readahead: add tracing event
[PATCH 09/11] readahead: add /debug/readahead/stats
I verified the new readahead behavior on various access patterns,
as well as stress tested the thrashing safety, by running 300 streams
with mem=128M.
Only 2031/61325=3.3% readahead windows are thrashed (due to workload
variation):
# cat /debug/readahead/stats
pattern readahead eof_hit cache_hit io sync_io mmap_io size async_size io_size
initial 20 9 4 20 20 12 73 37 35
subsequent 3 3 0 1 0 1 8 8 1
context 61325 1 5479 61325 6788 5 14 2 13
thrash 2031 0 1222 2031 2031 0 9 0 6
around 235 90 142 235 235 235 60 0 19
fadvise 0 0 0 0 0 0 0 0 0
random 223 133 0 91 91 1 1 0 1
all 63837 236 6847 63703 9165 0 14 2 13
And the readahead inside a single stream is working as expected:
# grep streams-3162 /debug/tracing/trace
streams-3162 [000] 8602.455953: readahead: readahead-context(dev=0:2, ino=0, req=287352+1, ra=287354+10-2, async=1) = 10
streams-3162 [000] 8602.907873: readahead: readahead-context(dev=0:2, ino=0, req=287362+1, ra=287364+20-3, async=1) = 20
streams-3162 [000] 8604.027879: readahead: readahead-context(dev=0:2, ino=0, req=287381+1, ra=287384+14-2, async=1) = 14
streams-3162 [000] 8604.754722: readahead: readahead-context(dev=0:2, ino=0, req=287396+1, ra=287398+10-2, async=1) = 10
streams-3162 [000] 8605.191228: readahead: readahead-context(dev=0:2, ino=0, req=287406+1, ra=287408+18-3, async=1) = 18
streams-3162 [000] 8606.831895: readahead: readahead-context(dev=0:2, ino=0, req=287423+1, ra=287426+12-2, async=1) = 12
streams-3162 [000] 8606.919614: readahead: readahead-thrash(dev=0:2, ino=0, req=287425+1, ra=287425+8-0, async=0) = 1
streams-3162 [000] 8607.545016: readahead: readahead-context(dev=0:2, ino=0, req=287436+1, ra=287438+9-2, async=1) = 9
streams-3162 [000] 8607.960039: readahead: readahead-context(dev=0:2, ino=0, req=287445+1, ra=287447+18-3, async=1) = 18
streams-3162 [000] 8608.790973: readahead: readahead-context(dev=0:2, ino=0, req=287462+1, ra=287465+21-3, async=1) = 21
streams-3162 [000] 8609.763138: readahead: readahead-context(dev=0:2, ino=0, req=287483+1, ra=287486+15-2, async=1) = 15
streams-3162 [000] 8611.467401: readahead: readahead-context(dev=0:2, ino=0, req=287499+1, ra=287501+11-2, async=1) = 11
streams-3162 [000] 8642.512413: readahead: readahead-context(dev=0:2, ino=0, req=288053+1, ra=288056+10-2, async=1) = 10
streams-3162 [000] 8643.246618: readahead: readahead-context(dev=0:2, ino=0, req=288064+1, ra=288066+22-3, async=1) = 22
streams-3162 [000] 8644.278613: readahead: readahead-context(dev=0:2, ino=0, req=288085+1, ra=288088+16-3, async=1) = 16
streams-3162 [000] 8644.395782: readahead: readahead-context(dev=0:2, ino=0, req=288087+1, ra=288087+21-3, async=0) = 5
streams-3162 [000] 8645.109918: readahead: readahead-context(dev=0:2, ino=0, req=288101+1, ra=288108+8-1, async=1) = 8
streams-3162 [000] 8645.285078: readahead: readahead-context(dev=0:2, ino=0, req=288105+1, ra=288116+8-1, async=1) = 8
streams-3162 [000] 8645.731794: readahead: readahead-context(dev=0:2, ino=0, req=288115+1, ra=288122+14-2, async=1) = 13
streams-3162 [000] 8646.114250: readahead: readahead-context(dev=0:2, ino=0, req=288123+1, ra=288136+8-1, async=1) = 8
streams-3162 [000] 8646.626320: readahead: readahead-context(dev=0:2, ino=0, req=288134+1, ra=288144+16-3, async=1) = 16
streams-3162 [000] 8647.035721: readahead: readahead-context(dev=0:2, ino=0, req=288143+1, ra=288160+10-2, async=1) = 10
streams-3162 [000] 8647.693082: readahead: readahead-context(dev=0:2, ino=0, req=288157+1, ra=288165+12-2, async=1) = 8
streams-3162 [000] 8648.221368: readahead: readahead-context(dev=0:2, ino=0, req=288168+1, ra=288177+15-2, async=1) = 15
streams-3162 [000] 8649.280800: readahead: readahead-context(dev=0:2, ino=0, req=288190+1, ra=288192+23-3, async=1) = 23
[...]
btw, Linus suggested to disable start-of-file readahead if lseek() has been called:
[PATCH 10/11] readahead: dont do start-of-file readahead after lseek()
At last, the updated context readahead will do more radix tree scans, so need
to optimize radix_tree_prev_hole():
[PATCH 11/11] radixtree: speed up next/prev hole search
It will on average reduce 8*64 level-0 slot searches to 32 level-0 slot
plus 8 level-1 node searches.
Thanks,
Fengguang
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