Subject: [linux-audio-dev] Re: Shared memory
From: Juhana Sadeharju (kouhia_AT_nic.funet.fi)
Date: Sat Jul 12 2003 - 22:32:54 EEST
Hello. I examined further the shared memory malfunction.
Locking the memory with mlockall() did not help: the shared
memory is not updated in the client. I used mlockall() both in
shmserver and in shmclient.
For some reason the shared memory is not a true shared memory.
The shmserver and shmclient do have the shared memory segment
obtained with the same shmid value, but processes seems to
have their own duplicates of it. I have no other explanation.
These duplicates were updated when the shmclient performed sleep(1).
Now I have non-waiting sched_yield() in the shmclient, and that helps.
shmserver is very computation intensive process: it goes through
the whole shared memory and increases the integers there. First
there are 250000 zeros, then 250000 ones, and so on. The client
reads only the first memory location in a loop, and prints out the
value if that value is not yet printed.
What practically happens is that shmserver has enough time to
increase the values multiple times before shmclient gets its
turn to check the values. shmclient does not print values such
as 1234, 1235, 1236 --- the scheduling period is too long for
that. In fact, the shmclient prints the values surprisingly
slow; maybe 20 times per second at maximum.
Without "renice 19" the shmclient prints values in steps of 24 (avg),
and with "renice 19" the shmclient prints values in steps of 2 (avg).
Maybe 20 times per second with "renice 19". Slow.
What this could mean? When an audio software does non-realtime
background computations, the other non-realtime processes could be
in trouble even if they run at different user priorities.
These processes could be a disk server, a GUI, etc.
Question: does the same "shared memory" problem occur with threads?
Intuitively there should not be problems, but never know after what
I have seen.
Best regards,
Juhana
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