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+From kaf@nwlink.com Fri Apr 26 14:22:39 2002
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+ Fri, 26 Apr 2002 11:18:00 -0700
+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>
+References: <15561.26116.817541.950416@doppelbock.patentinvestor.com>
+ <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.
+
projects / users / rhaas / postgres.git / commitdiff