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Bruce Momjian 23 years ago
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Date: Thu, 6 Mar 2003 22:04:12 -0800
From: Sean Chittenden <sean@chittenden.org>
To: Neil Conway <neilc@samurai.com>
cc: Tom Lane <tgl@sss.pgh.pa.us>,
Christopher Kings-Lynne <chriskl@familyhealth.com.au>,
PostgreSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: [PERFORM] [COMMITTERS] pgsql-server/ /configure /configure.in rc/incl ...
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> > I don't have my copy of Steven's handy (it's some 700mi away atm
> > otherwise I'd cite it), but if Tom or someone else has it handy, look
> > up the example re: the performance gain from read()'ing an mmap()'ed
> > file versus a non-mmap()'ed file. The difference is non-trivial and
> > _WELL_ worth the time given the speed increase.
>=20
> Can anyone confirm this? If so, one easy step we could take in this
> direction would be adapting COPY FROM to use mmap().
Weeee! Alright, so I got to have some fun writing out some simple
tests with mmap() and friends tonight. Are the results interesting?
Absolutely! Is this a simple benchmark? Yup. Do I think it
simulates PostgreSQL? Eh, not particularly. Does it demonstrate that
mmap() is a win and something worth implementing? I sure hope so. Is
this a test program to demonstrate the ideal use of mmap() in
PostgreSQL? No. Is it a place to start a factual discussion? I hope
so.
I have here four tests that are conditionalized by cpp.
# The first one uses read() and write() but with the buffer size set
# to the same size as the file.
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -o test-=
mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047013002.412516
Time: 82.88178
Completed tests
82.09 real 2.13 user 68.98 sys
# The second one uses read() and write() with the default buffer size:
# 65536
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -o test-mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is default read size: 65536
Number of iterations: 100000
Start time: 1047013085.16204
Time: 18.155511
Completed tests
18.16 real 0.90 user 14.79 sys
# Please note this is significantly faster, but that's expected
# The third test uses mmap() + madvise() + write()
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -o test-mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047013103.859818
Time: 8.4294203644
Completed tests
7.24 real 0.41 user 5.92 sys
# Faster still, and twice as fast as the normal read() case
# The last test only calls mmap()'s once when the file is opened and
# only msync()'s, munmap()'s, close()'s the file once at exit.
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -DDO_MMAP_ONCE=3D1 -o test-mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047013111.623712
Time: 1.174076
Completed tests
1.18 real 0.09 user 0.92 sys
# Substantially faster
Obviously this isn't perfect, but reading and writing data is faster
(specifically moving pages through the VM/OS). Doing partial writes
from mmap()'ed data should be faster along with scanning through
mmap()'ed portions of - or completely mmap()'ed - files because the
pages are already loaded in the VM. PostgreSQL's LRU file descriptor
cache could easily be adjusted to add mmap()'ing of frequently
accessed files (specifically, system catalogs come to mind). It's not
hard to figure out how often particular files are accessed and to
either _avoid_ mmap()'ing a file that isn't accessed often, or to
mmap() files that _are_ accessed often. mmap() does have a cost, but
I'd wager that mmap()'ing the same file a second or third time from a
different process would be more efficient. The speedup of searching
through an mmap()'ed file may be worth it, however, to mmap() all
files if the system is under a tunable resource limit
(max_mmaped_bytes?).
If someone is so inclined or there's enough interest, I can reverse
this test case so that data is written to an mmap()'ed file, but the
same performance difference should hold true (assuming this isn't a
write to a tape drive ::grin::).
The URL for the program used to generate the above tests is at:
http://people.freebsd.org/~seanc/mmap_test/
Please ask if you have questions. -sc
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Sean Chittenden
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From: Sean Chittenden <sean@chittenden.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Neil Conway <neilc@samurai.com>,
Christopher Kings-Lynne <chriskl@familyhealth.com.au>,
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> > Absolutely! Is this a simple benchmark? Yup. Do I think it
> > simulates PostgreSQL? Eh, not particularly.
I think quite a few of these Q's would have been answered by reading
the code/Makefile....
> This would be on what OS?
FreeBSD, but it shouldn't matter. Any reasonably written VM should
have similar numbers (though BSD is generally regarded as having the
best VM, which, I think Linux poached not that long ago, iirc
::grimace::).
> What hardware?
My ultra-pathetic laptop with some fine - overly-noisy and can hardly
buildworld - IDE drives.
> What size test file?
In this case, only 72K. I've just updated the test program to use an
array of files though.
> Do the "iterations" mean so many reads of the entire file, or so
> many buffer-sized read requests?
In some cases, yes. With the file mmap()'ed, sorta. One of the test
cases (the one that did it in ~8s), mmap()'ed and munmap()'ed the file
every iteration and was twice as fast as the vanilla read() call.
> Did the mmap case actually *read* anything, or just map and unmap
> the file?
Nope, read it and wrote it out to stdout (which was redirected to
/dev/null).
> Also, what did you do to normalize for the effects of the test file
> being already in kernel disk cache after the first test?
That honestly doesn't matter too much since I wasn't testing the rate
of reading in files from my hard drive, only the OS's ability to
read/write pages of data around. In any case, I've updated my test
case to iterate through an array of files instead of just reading in a
copy of /etc/services. My laptop is generally a poor benchmark for
disk read performance given it takes 8hrs to buildworld, over 12hrs to
build mozilla, 18 for KDE, and about 48hrs for Open Office. :)
Someone with faster disks may want to try this and report back, but it
doesn't matter much in terms of relevancy for considering the benefits
of mmap(). The point is that there are calls that can be used that
substantially speed up read()'s and write()'s by allowing the VM to
align pages of data and give hints about its usage. For the sake of
argument re: the previously done tests, I'll reverse the order in
which I ran them and I bet dime to dollar that the times will be
identical.
% make =
~/open_source/mmap_test
cp -f /etc/services ./services
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -DDO_MMAP_ONCE=3D1 -o mmap-test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047064672.276544
Time: 1.281477
Completed tests
1.29 real 0.10 user 0.92 sys
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -o mmap-test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047064674.266191
Time: 7.486622
Completed tests
7.49 real 0.41 user 6.01 sys
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -o mmap-test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is default read size: 65536
Number of iterations: 100000
Start time: 1047064682.288637
Time: 19.35214
Completed tests
19.04 real 0.88 user 15.43 sys
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -o mmap-=
test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047064701.867031
Time: 82.4294540875
Completed tests
81.57 real 2.10 user 69.55 sys
Here's the updated test that iterates through. Ooh! One better, the
files I've used are actual data files from ~pgsql. The new benchmark
iterates through the list of files and and calls bench() once for each
file and restarts at the first file after reaching the end of its
list (ARGV).
Whoa, if these tests are even close to real world, then we at the very
least should be mmap()'ing the file every time we read it (assuming
we're reading more than just a handful of bytes):
find /usr/local/pgsql/data -type f | /usr/bin/xargs /usr/bin/time ./mmap-te=
st > /dev/null
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047071143.463360
Time: 12.109530
Completed tests
12.11 real 0.36 user 6.80 sys
find /usr/local/pgsql/data -type f | /usr/bin/xargs /usr/bin/time ./mmap-te=
st > /dev/null
Page size: 4096
File read size is default read size: 65536
Number of iterations: 100000
.... [been waiting here for >40min now....]
Ah well, if these tests finish this century, I'll post the results in
a bit, but it's pretty clearly a win. In terms of the data that I'm
copying, I'm copying ~700MB of data from my test DB on my laptop. I
only have 256MB of RAM so I can pretty much promise you that the data
isn't in my system buffers. If anyone else would like to run the
tests or look at the results, please check it out:
o1 and o2 should be the only targets used if FILES is bigger than the
RAM on the system. o3's by far and away the fastest, but only in rare
cases will a DBA have more RAM than data. But, as mentioned earlier,
the LRU cache could easily be modified to munmap() infrequently
accessed files to keep the size of mmap()'ed data down to a reasonable
level.
The updated test programs are at:
http://people.FreeBSD.org/~seanc/mmap_test/
-sc
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Sean Chittenden
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