Date: Sat, 18 Oct 2008 18:12:02 -0500 From: Dan Nelson <dnelson@allantgroup.com> To: Ivan Voras <ivoras@freebsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Pipes, cat buffer size Message-ID: <20081018231201.GM99270@dan.emsphone.com> In-Reply-To: <gddmip$im0$1@ger.gmane.org> References: <gddjoj$apg$1@ger.gmane.org> <20081018213502.GL99270@dan.emsphone.com> <gddmip$im0$1@ger.gmane.org>
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In the last episode (Oct 19), Ivan Voras said: > Dan Nelson wrote: > > In the last episode (Oct 18), Ivan Voras said: > >> I'm working on a program that's intended to be used as a "filter", > >> as in "something | myprogram > file". I'm trying it with cat and > >> I'm seeing my read()s return small blocks, 64 kB in size. I > >> suppose this is because cat writes in 64 kB blocks. So: > >> > >> a) Is there a way to programatically, per-process, set the pipe buffer > >> size? The program in question is a compressor and it's particularly > >> inefficient when given small blocks and I'm wondering if the system can > >> buffer enough data for it. > > > > Why not keep reading until you reach your desired compression block > > size? Bzip2's default blocksize is 900k, for example. > > Of course. But that's not the point :) From what I see (didn't look at > the code), Linux for example does some kind of internal buffering that > decouples how the reader and the writer interact. I think that with > FreeBSD's current behaviour the writer could write 1-byte buffers and > the reader will be forced to read each byte individually. I don't know > if there's some ulterior reason for this. No; take a look at /sys/kern/sys_pipe.c . Depending on how much data is in the pipe, it switches between async in-kernel buffering (<8192 bytes), and direct page wiring between sender and receiver (basically zero-copy). > >> b) Is there any objection to the following patch to cat: > > > > It might be simpler to just use "dd if=myfile obs=1m" instead of > > patching cat. > > I believe patching cat to bring its block size into the century of the > fruitbat has its own benefits. -- Dan Nelson dnelson@allantgroup.com
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