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SubjectRe: [PATCH] livelock in elevator scheduling
Jens Axboe writes:
> > The benchmark invokes lots of processes, each corresponds to a client,
> > and each accesses different portion of few large files. We have
> > enough memory to hold all dirty data at onece (1GB without himem), so
> > if no I/O blocking occur, all process can be run simultaneously with
> > limited amount of dirty flush I/O stream.
>
> Flushing that much dirty data will always end up blocking waiting
> for request slots.

Yes, such benchmarks need moderate long startup time, like 10 minutes
or more. During that period, physical read requests are issued and
those are waiting on a queue, resulting low performance value.
As file data is accumulated into memory, CPU usage goes
high. Finally, all required data are on the buffer, the system can use
100% CPU.

I show you the example of "vmstat 10" during the test.
It includes the beginning, startup, full-usage states.

At the current setting, the hard-threshold of dirty cache, which needs
synchronous flushing, is over 300MB and this test doesn't reach at the
limit.

During startup period, "cache" steadily increase, but until READ
is disappeared cpu usage stays very low.

If some of the read requests have inferior priority to others, those
requests need (extremely) long time to be served, then the startup
process never end in reasonable time.


procs memory swap io system cpu
r b wswpd free buff cache si so bi bo in cs us sy id
0 0 0 0 863424 1760 13200 0 0 0 0 101 6 0 0 100

START CLIENTS
0 16 0 0 843448 2052 25796 0 0 322 0 587 458 1 2 97
0 16 0 0 827988 2052 40836 0 0 376 0 619 491 1 0 99
0 16 0 0 813276 2052 55120 0 0 357 0 630 489 1 0 98

DIRTY FLUSH STARTS
0 16 0 0 802488 2052 65608 0 0 262 34 541 420 1 0 99
9 more lines
0 16 0 0 717824 2052 148040 0 0 151 73 572 442 1 0 99
9 more lines
0 16 1 0 657216 2052 206980 0 0 119 71 790 602 2 1 98
9 more lines
0 16 1 0 652484 2052 211592 0 0 115 72 815 613 2 0 98
9 more lines
0 16 1 0 623256 2052 240072 0 0 20 119 432 424 1 0 99
9 more lines
0 16 1 0 598600 2052 264088 0 0 16 119 884 719 2 1 97
9 more lines
1 15 1 0 592156 2052 270332 0 0 15 119 1609 1185 5 1 94
0 16 1 0 591540 2052 270916 0 0 15 119 1475 1102 4 1 95
1 15 1 0 590920 2052 271512 0 0 15 119 1669 1243 5 1 94
1 15 1 0 590344 2052 272072 0 0 14 122 2091 1500 7 2 92
0 16 1 0 589752 2052 272644 0 0 14 120 2484 1767 8 2 91
1 15 1 0 589180 2052 273196 0 0 14 120 2674 1905 8 2 90
2 14 1 0 588608 2052 273748 0 0 14 122 3059 2142 10 2 88
0 16 1 0 588044 2052 274288 0 0 14 122 4430 3037 16 3 82
0 16 1 0 587524 2052 274800 0 0 13 123 5892 3913 21 4 75
1 14 2 0 587036 2052 275236 0 0 11 121 8525 6062 31 7 62
10 4 2 0 586688 2052 275576 0 0 9 124 12162 8902 47 10 43
14 1 2 0 586528 2052 275708 0 0 3 129 17605 7710 80 18 2

FULL CPU USAGE (No physical read request is issued)
15 0 2 0 586492 2052 275740 0 0 1 131 18843 6652 82 18 0
14 0 2 0 586484 2052 275748 0 0 0 132 19210 6340 82 18 0

CONTINUE TO THE END

--
Computer Systems Laboratory, Fujitsu Labs.
kumon@flab.fujitsu.co.jp
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