Strange mnesia behaviour

Anders Nygren <>
Mon Aug 25 01:38:50 CEST 2003


Hi
I have found a strange thing in mnesia. I was trying
to do some throughput measurement on a process.

My process does the following:
1, receive one message
2, make one mnesia:select
3, send one message

My test was to send N identicat messages to my process
and measure the time it took to process all messages.
To my suprise the times grew non-linearly with N.
eprof shows that
N = 10 - > mnesia:flush_downs/0 is called 55 times
N = 100 -> mnesia:flush_downs/0 is called 5000 times
N = 4000 -> a whopping 4 million times.

Also if I run the test twice with 50 messages I get
2500 mnesia:flush_downs, but when I run one test with
100 messages there are 5000.

I the mnesia table has one record.
I am using a single node -> no distribution.
The table is type bag, disc_copy.
I am using R9C on Suse 8.2.

Is there something wrong with mnesia or is there 
something I dont understand?

/Anders

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