From: | Markus Innerebner <markus(dot)innerebner(at)inf(dot)unibz(dot)it> |
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To: | Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at> |
Cc: | <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Question about caching on full table scans |
Date: | 2012-08-30 17:34:56 |
Message-ID: | 7FB562B8-75CB-446A-B69F-6D999D171710@inf.unibz.it |
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Thread: | |
Lists: | pgsql-performance |
Hi Laurenz,
>
> In your approach 1 to 3, what do you mean with "load into main memory"?
I forgot to say: I use Java and connect with JDBC.
in approach 1 I do an initial loading of the entire relation, by executing 1 SQL query to load all edges in main memory, where I create my main memory structure
as an adjacency list.
> Do you
> a) make sure that the data you talk about are in the PostgreSQL buffer
> cache
> or
> b) retrieve the data from PostgreSQL and store it somewhere in your
> application?
In approach 1 I do that, as described before.
But after each experiment I restart a new java process.
>
> To clear PostgreSQL's cache, restart the server.
> That should be a fast operation.
> Since version 8.3, PostgreSQL is smart enough not to evict the
> whole cache for a large sequential scan.
>
> To flush the filesystem cache (from Linux 2.6.16 on), use
> sync; echo 3 > /proc/sys/vm/drop_caches
I started to do that , and
yes, this solves my problem!!
I assume that deleting file system cache implies that also postgres cache is deleted, isn't it ?
so i will invoke after each experiment this command
thanks a lot!!
Markus
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