linux-kernel - Re: pipe(2), read/write, maximums and behavior.

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Date:	Mon, 06 Jul 2009 11:00:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Linda Walsh <lkml@...nx.org>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: pipe(2), read/write, maximums and behavior.

Linda Walsh a écrit :
> I've seen a few shells claim to limit pipe sizes to 8 512Byte buffers.
> Don't know where they get this value or how they think it applies, but
> it certainly doesn't seem to apply in linux.   However, I'm not
> sure what limits do apply compared to available memory.
> I suppose, starting off, one might look at at a maximum of
> (Physical+Swap-resident-non-swappable mem)/2 as a top limit.
> 
> A test machine I have has 8GB physical memory with a bit over 4GB
> of swap space making for about 12GB of memory.
> 
> If total memory was to go toward my proglet that splits into a master
> writer and slave pipe reader, they'd have to split memory to have
> matching buffer read/write sizes. I'd "expect", (I think) at least
> a 2GB write/read to work, and possibly a 4GB write/read to work
> with alot of swap activity -- that's assuming there are no other
> restraints in dividing 12GB of address space.
> 
> As it turns out -- the program dies at 2GB (the 1GB write/read works)
> but when the program tries a 2GB write & read it refuses the full write
> and the child gets less than 2GB.
> 
> The master gets back that it wrote 2097148KB, though it tried to
> write 2097152KB (and the child receives the 2GB-4K buffer upon read).
> 
> This is on a x86_64 machine, and unsigned long values are 8-bytes
> wide and being used with the read and write calls for lengths.
> 
> Shouldn't a 2GB read/write work?  At most, together the master
> and slave would have only used 4GB for each to have a 2GB buffer.
> 
> How would one determine the maximum size for 1 huge read or write
> through the pipe (from the pipe system call)?
> 
> On 2GHz multi-core machines, I get about 512MB/s throughput.
> 
> I attached the source file so anyone can see my methodology.
> 
> you have to include "-lrt" on the gcc command line as it uses
> clock_gettime to estimate the time for the write call (the read
> call always comes back with values too small to be reasonable, so
> I don't bother printing them.
> 
> 
> 

read()/write() system calls use generic vfs_read()/vfs_write() calls,
that in turn use rw_verify_area() which limits 'count' of bytes
to MAX_RW_COUNT

#define MAX_RW_COUNT (INT_MAX & PAGE_CACHE_MASK)

So yes, this currently limits to 2GB - (PAGE_SIZE)  (PAGE_SIZE=4KB on i386),
even on x86_64 kernels.

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