[mapserver-dev] Trying to improve perfs of MapServer with PostGIS
Paul Ramsey
pramsey at cleverelephant.ca
Mon Dec 14 11:15:12 PST 2015
Please bear in mind that the original implementation using binary
cursors was deliberately *removed* (by me) because it added so much
complexity to the code base. (In terms of having to maintain a correct
transaction state throughout the query lifecycle.)
However, your patch is very small, I see. Perhaps pgsql support of
binary has improved a lot since the days of binary cursors. (Which
could only be used in transaction, at the time.)
Adding TWKB to mapnik has substantially improved network performance [1] for us.
http://blog.cartodb.com/smaller-faster/
Removing precision in map rendering works like a charm, I'd highly
recommend it. However, Mapserver's dual nature as both a feature and
map server would make a change to support that more tricky than the
Mapnik change.
The only issue I'd have to this being a PR is some careful research as
to when this RESULTSET_TYPE flag was added to PQexecParams I suppose
we could have missed it all those years ago, and thus used binary
cursors, but it could also be an add-on as of a certain date. Knowing
when it was added would be good. (Probably long enough ago that we
don't even need to #ifdef a version test on it, but I would like
confirmation.)
ATB,
P
On Mon, Dec 14, 2015 at 7:10 AM, Patrick Valsecchi
<patrick.valsecchi at camptocamp.com> wrote:
> Daniel: I do agree with your analysis.
>
> Even: That is a bit out of scope for this patch and I'm no fan of reducing
> the precision, even for WMS.
>
> On Fri, Dec 11, 2015 at 5:06 PM, Even Rouault <even.rouault at spatialys.com>
> wrote:
>>
>> Le vendredi 11 décembre 2015 13:38:56, Daniel Morissette a écrit :
>> > Hi Patrick,
>> >
>> > We have run into similar behaviors with a quite similar setup on Amazon
>> > a little while ago, in our case we had a dedicated DB server with SSD
>> > (like yours) and multiple rendering nodes connecting to it, and adding
>> > more nodes or more powerful ones still resulted in a constant throughput
>> > and the additional rendering nodes CPU running mostly idle. After much
>> > hair pulling we finally found out that we were hitting the 1 Gbps limit
>> > of the network connection between the DB and rendering node.
>> >
>> > I suspect you are running into the same thing here: if you could verify
>> > that the size of the original vs binary response is about the double,
>> > then that would confirm this possibility. We used PGBouncer to look at
>> > the volume of data transferred by the DB server, it produces output like
>> > this:
>> >
>> > 2014-06-06 18:49:44.597 27891 LOG Stats: 852 req/s, in 435114 b/s, out
>> > 83424883 b/s,query 7133 us
>> > 2014-06-06 18:50:44.597 27891 LOG Stats: 818 req/s, in 418046 b/s, out
>> > 79177139 b/s,query 5761 us
>> > 2014-06-06 18:52:44.598 27891 LOG Stats: 898 req/s, in 458041 b/s, out
>> > 88441708 b/s,query 7949 us
>> >
>> >
>> > Then to really confirm it, look at the number of hits per second in each
>> > test case (instead of the processing time per request). You will likely
>> > notice that it is constant for a given zoom level once you hit the
>> > network limit no matter how many parallel requests you have. This is
>> > because the number of hits per seconds that can be served are limited by
>> > the volume of data per second that the DB server can deliver to
>> > MapServer (around ~800Mbits/sec for a 1 Gbps connection).
>> >
>> > In our case, if I remember correctly, we used to hit a limit of about 40
>> > maps/second, where each map was averaging 2MB of data... so we were
>> > capping at 4 hits x 2MB ~= 80MBytes per second from the DB (or
>> > ~800Mbits/sec) ... which is as I wrote above in line with a 1Gbps limit.
>>
>> If bandwidth is the bottleneck, an even more efficient solution than the
>> binary
>> WKB representation would be to use http://postgis.net/docs/manual-
>> dev/ST_AsTWKB.html . This would require a TWKB decoder to be developped in
>> MapServer and be likely only appropriate for WMS like scenarios where you
>> can
>> afford loosing some precision to coordinates values.
>>
>> Even
>>
>> >
>> > Good luck!
>> >
>> > Daniel
>> >
>> > On 2015-12-11 4:13 AM, Patrick Valsecchi wrote:
>> > > Hi,
>> > >
>> > > I was a bit disapointed by the perfs of reading the features from
>> > > PostGIS compared to reading them from a local Shapefile. So I've
>> > > pocked
>> > > my nose around the code and I saw that geometries were fetched from
>> > > the
>> > > DB in EWKB encoded in hex.
>> > >
>> > > So I went ahead and changed the code to use libpq's binary mode when
>> > > we
>> > > are fetching features. To avoid conversion nightmares when fetching
>> > > attributes, I'm tricking Postgres into sending them in their textual
>> > > form. That way, the behavior of MapServer should not change regarding
>> > > the other columns.
>> > >
>> > > The patch can be found in github [1].
>> > >
>> > > General testing algo:
>> > > for each zoom level
>> > > start at the same time N threads that does
>> > > for each iteration
>> > > compute random BBOX
>> > > get the 1000x1000 image (throw the content out, but read it)
>> > > Wait for all the threads to stop
>> > > Collect the stats from the threads (throwing out the slowest result
>> > > and
>> > > the fastest result).
>> > >
>> > > The DB is on a db.m4.large from Amazon with SSD disk. Mapserver is on
>> > > apache with fcgid (up to 20 processes) and runs on a m4.xlarge. The
>> > > measures are taken from another Amazon machine to avoid hitting the
>> > > bandwidth limit too fast.
>> > >
>> > > Times are for one query from one thread and are given in milliseconds
>> > > along with their standard deviation. Each time, I did a full run
>> > > before
>> > > taking measures.
>> > >
>> > > Data used is the swiss villages (total 2479 features) with polygon
>> > > contours in a 1.7MB shapefile file [2], imported as is in PostGIS.
>> > >
>> > > nbThreads=1 nbIterations=20
>> > > zoom level 1.00 4.00 16.00 32.00
>> > > original 964± 14 222± 126 66± 18 68± 19
>> > > binary 807± 13 194± 111 87± 28 79± 24
>> > > shapefile 554± 94 187± 107 72± 26 56± 3
>> > >
>> > >
>> > > nbThreads=5 nbIterations=20
>> > > zoom level 1.00 4.00 16.00 32.00
>> > > original 3686± 946 403± 264 84± 37 70± 22
>> > > binary 1710± 486 340± 242 105± 59 89± 32
>> > > shapefile 519± 225 278± 166 91± 58 80± 34
>> > >
>> > > nbThreads=10 nbIterations=20
>> > > zoom level 1.00 4.00 16.00 32.00
>> > > original 7287±1936 800± 575 119± 79 110± 81
>> > > binary 3737± 647 471± 294 123± 70 110± 54
>> > > shapefile 884± 241 412± 269 111± 119 98± 57
>> > >
>> > > nbThreads=20 nbIterations=20
>> > > zoom level 1.00 4.00 16.00 32.00
>> > > original 14969±2507 1643±1221 239± 231 166± 103
>> > > binary 7649± 730 857± 576 210± 121 181± 77
>> > > shapefile 1455± 438 483± 326 143± 97 126± 75
>> > >
>> > > What is what:
>> > > * original: Mapserver 7.0 (git 157fa47)
>> > > * binary: Same as above with a small patch to configure libpg to use
>> > >
>> > > binary transfer.
>> > >
>> > > * shapefile: Same as original, but using a shapefile on the local
>> > > disk
>> > >
>> > > (just here for comparison).
>> > >
>> > > We can see that when the machine gets parallel queries, we quickly get
>> > > a
>> > > factor 2 in perfs when there are a lot of features (low zoom level).
>> > > There is no measurable negative impact at higher zoom levels and lower
>> > > loads.
>> > >
>> > > Now, what do you guys think? Do you see in risk? Or should do a pull
>> > > request?
>> > >
>> > > Thanks.
>> > >
>> > >
>> > > [1]
>> > >
>> > > https://github.com/pvalsecc/mapserver/commit/45bd3d5795c9108618c37cc8c747
>> > > 2809cff54d16 [2]
>> > >
>> > > http://www.swisstopo.admin.ch/internet/swisstopo/en/home/products/landsca
>> > > pe/swissBOUNDARIES3D.html
>> > >
>> > >
>> > >
>> > > _______________________________________________
>> > > mapserver-dev mailing list
>> > > mapserver-dev at lists.osgeo.org
>> > > http://lists.osgeo.org/mailman/listinfo/mapserver-dev
>>
>> --
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>> http://www.spatialys.com
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