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Re: port-vax/41139: pipe(2) seems broken on VAX

The following reply was made to PR port-vax/41139; it has been noted by GNATS.

From: "Michael L. Hitch" <>
Subject: Re: port-vax/41139: pipe(2) seems broken on VAX
Date: Tue, 7 Apr 2009 14:48:39 -0600 (MDT)

    The problem appears to only occur when the requested transfer size is > 
 PIPE_SIZE, and an additional buffer of BIG_PIPE_SIZE is allocated and 
 used.  The defaults are 16KB and 64KB respectively.  Setting BIG_PIPE_SIZE 
 to a smaller value (I've tested 63KB) seems to cure the problem.  The real 
 root of the problem is that sys_pipe.c uses uiomove() to transfer the 
 data, which uses copyin()/copyout().  On the vax, this is done with the 
 movc3 instruction, which uses a 16-bit length, and will not transfer 
 anything when the length is 0 (which it will be with a 64KB size).
    The simplest fix would be to set BIG_PIPE_SIZE in 
 sys/arch/vax/include/param.h.  There does appear to be an improvment 
 between using 32KB and 63KB in the one test I did.
    A better fix would be to make copyin()/copyout() work with 64KB or 
 larger sizes.  I'm not sure what performance impact that might have 
 though.  The quick test I just did shows that just fixing copyin/copyout 
 to check for 64KB and transfer one byte with movb before doing the movc3 
 seems to show that it was slightly slower than when I was using a 63KB 
    One other note - this also appears to have been the problem with using 
 scp on the vax to transfer files.  My scp copies no longer get corrupted 
 data.  [Since scp does a pipe to run the remote ssh command and copies the 
 data from the pipe, this makes sense.]
 Michael L. Hitch             
 Computer Consultant
 Information Technology Center
 Montana State University       Bozeman, MT     USA

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