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From kaf@nwlink.com Fri Apr 26 14:22:39 2002

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From: Kyle <kaf@nwlink.com>

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Message-ID: <15561.39384.296503.501888@doppelbock.patentinvestor.com>

Date: Fri, 26 Apr 2002 11:18:00 -0700

To: Bruce Momjian <pgman@candle.pha.pa.us>

Subject: Re: [HACKERS] Sequential Scan Read-Ahead

In-Reply-To: <200204261444.g3QEiFh11090@candle.pha.pa.us>

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<200204261444.g3QEiFh11090@candle.pha.pa.us>

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Status: ORr

Hey Bruce,

I'll forward this to the list if you think they'd benefit from it.

I'm not sure it says anything about read-ahead, I think this is more a

kernel caching issue. But I've been known to be wrong in the past.

Anyway...

the test:

foreach i (5 15 20 25 30 )

echo $i

time dd bs=8k if=big_file1 of=/dev/null &

sleep $i

time dd bs=8k if=big_file1 of=/dev/null &

wait

end

I did a couple more runs in the low range since their is a drastic

jump in elapsed (wall clock) time after doing a 6 second sleep:

first process second process

sleep user kernel elapsed user kernel elapsed

0 sec 0.200 7.980 1:26.57 0.240 7.720 1:26.56

3 sec 0.260 7.600 1:25.71 0.260 8.100 1:22.60

5 sec 0.160 7.890 1:26.04 0.220 8.180 1:21.04

6 sec 0.220 8.070 1:19.59 0.230 7.620 1:25.69

7 sec 0.210 9.270 1:57.92 0.100 8.750 1:50.76

8 sec 0.240 8.060 4:47.47 0.300 7.800 4:40.40

15 sec 0.200 8.500 4:51.11 0.180 7.280 4:44.36

20 sec 0.160 8.040 4:40.72 0.240 7.790 4:37.24

25 sec 0.170 8.150 4:37.58 0.140 8.200 4:33.08

30 sec 0.200 7.390 4:37.01 0.230 8.220 4:31.83

with a sleep of > 6 seconds, either the second process isn't getting

cached data or readahead is being turned off. I'd guess the former, I

don't see why read-ahead would be turned off since they're both doing

sequential operations.

Although with 512mb of memory and the disk reading at about 22 mb/sec,

maybe we're not hitting the cache. I'd guess at least ~400 megs of

kernel cache is being used for buffering this 2 gig file. free(1)

reports:

% free

total used free shared buffers cached

Mem: 512924 508576 4348 0 2640 477960

-/+ buffers/cache: 27976 484948

Swap: 527152 15864 511288

so shouldn't we be getting cached data even with a sleep of up to

about (400/22) 18 seconds...? Maybe I'm just in the dark on what's

really happening. I should point out that this is linux 2.4.18.

Bruce Momjian wrote:

>

> I am trying to illustrate how kernel read-ahead could be turned off in

> certain cases.

>

> ---------------------------------------------------------------------------

>

> Kyle wrote:

> > What are you trying to test, the kernel's cache vs disk speed?

> >

> >

> > Bruce Momjian wrote:

> > >

> > > Nice test. Would you test simultaneous 'dd' on the same file, perhaps

> > > with a slight delay between to the two so they don't read each other's

> > > blocks?

> > >

> > > seek() in the file will turn off read-ahead in most OS's. I am not

> > > saying this is a major issue for PostgreSQL but the numbers would be

> > > interesting.