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From: Rich <rich@example.invalid>
Newsgroups: comp.os.linux.misc
Subject: Re: Are We Back to the "Wars" Now ?
Date: Sun, 24 Nov 2024 16:56:32 -0000 (UTC)
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Lew Pitcher <lew.pitcher@digitalfreehold.ca> wrote:
> On Sun, 24 Nov 2024 14:25:10 +0000, Rich wrote:
> 
>> 186282@ud0s4.net <186283@ud0s4.net> wrote:
>>> On 11/23/24 4:25 PM, Lawrence D'Oliveiro wrote:
>>>> That’s irrelevant.  Pipes originated on the earliest Unix machine, 
>>>> which was a PDP-11 with only a 64kiB address space.  They work 
>>>> great for pumping around gigabytes of data, but you don’t need 
>>>> gigabyte-sized memory buffers to do that.
>>> 
>>>   It all has to be SOMEWHERE ... if not in RAM then
>>>   on a mass storage device.
>> 
>> Nope, at least not with pipes.
> 
> Hold on a sec....  pipes are /buffered/ in RAM, so there's at least a 
> small bit of ram set aside for each open pipe.  On Linux, pipe(7) 
> says "In Linux versions before 2.6.11, the capacity of a pipe was the 
> same as the system page size (e.g., 4096 bytes on i386).  Since Linux 
> 2.6.11, the pipe capacity is 65536 bytes.  Since Linux 2.6.35, the 
> default pipe capacity is 65536 bytes, but the capacity can be queried 
> and set using the fcntl(2) F_GETPIPE_SZ and F_SETPIPE_SZ operations.  
> See fcntl(2) for more information." and that "capacity" referred to 
> consists of a kernel-managed RAM buffer.
> 
> [snip]

Fair enough -- about 64k of RAM per each pipe is used for the kernel 
FIFO buffer.

But that is at most 64k of RAM (unless one's C code adjusts the buffer 
size), regardless of how many GiB flow through the pipe.  So it "all" 
(to quote the nymshifter) does not have to be in RAM.