Re: [PATCH] increase pipe size/buffers/atomicity :D

[Brian Haslett <knotwurk@gmail.com>]

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> On Wed, 2010-04-07 at 19:38 -0600, brian wrote:
>> (tested and working with 2.6.32.8 kernel, on a Athlon/686)
>
> It would be good to know what issue this addresses. Gives people a way
> to weigh any side-effects/drawbacks against the benefits, and an
> opportunity to suggest alternate/better approaches.
>

I wouldn't say it addresses anything that I'd really consider broken;
it started as a personal experiment of mine, aimed at some little
performance gain.  I figured, hey, bigger pipes, why not? Looks like
these pipe sizes have practically been around since the epoch.


>>  #define PIPE_BUF_FLAG_LRU      0x01    /* page is on the LRU */
>>  #define PIPE_BUF_FLAG_ATOMIC   0x02    /* was atomically mapped */
>> --- include/asm-generic/page.h.orig     2010-04-06 22:57:08.000000000
>> -0500
>> +++ include/asm-generic/page.h  2010-04-06 22:57:23.000000000 -0500
>> @@ -12,7 +12,7 @@
>>
>>  /* PAGE_SHIFT determines the page size */
>>
>> -#define PAGE_SHIFT     12
>> +#define PAGE_SHIFT     13
>
> This has pretty wide-ranging implications, both within and across
> arches. I don't think it's something that can be changed easily. Also I
> don't believe this #define used in your configuration (Athlon/686)
> unless you're running without a MMU.
>

actually, the reason I went after this, gets into the only reason I
started this whole ordeal to begin with, line#135 in pipe_fs_i.h that
reads "#define PIPE_SIZE    PAGE_SIZE".


>>  #ifdef __ASSEMBLY__
>>  #define PAGE_SIZE      (1 << PAGE_SHIFT)
>>  #else
>> --- include/linux/limits.h.orig 2010-04-06 22:54:15.000000000 -0500
>> +++ include/linux/limits.h      2010-04-06 22:56:28.000000000 -0500
>> @@ -10,7 +10,7 @@
>>  #define MAX_INPUT        255   /* size of the type-ahead buffer */
>>  #define NAME_MAX         255   /* # chars in a file name */
>>  #define PATH_MAX        4096   /* # chars in a path name including nul */
>> -#define PIPE_BUF        4096   /* # bytes in atomic write to a pipe */
>> +#define PIPE_BUF        8192   /* # bytes in atomic write to a pipe */
>

You'd think so (according to some posts I'd read before I tried this),
but I actually tried several variations on a few things, and until I
changed *this one in particular*, my kernel would in fact boot up
fine, but the shell/init/system phase itself would start giving me
errors to the effect of "unable to create pipe" and "too many file
descriptors open" over and over again.

>> --- include/linux/pipe_fs_i.h.orig      2010-04-06 22:56:51.000000000
>> -0500
>> +++ include/linux/pipe_fs_i.h   2010-04-06 22:56:58.000000000 -0500
>> @@ -3,7 +3,7 @@
>>
>>  #define PIPEFS_MAGIC 0x50495045
>>
>> -#define PIPE_BUFFERS (16)
>> +#define PIPE_BUFFERS (32)
>
> This worries me. In several places there are functions with 2 or 3
> pointer arrays of dimension [PIPE_BUFFERS] on the stack. So this adds
> anywhere from 128 to 384 bytes to the stack in these functions depending
> on sizeof(void*) and the number of arrays.
>

As my initial hope/goal was to just increase the size of the pipes, I
figured I may as well increase the buffers as well (although I'll
admit ignorance to not having poked around every little .c/.h file
that calls it).

I guess I wasn't seriously trying to push anyone into jumping through
hoops for this thing, I was just a little excited and figured I'd
share with you all.  I probably spent the better part of a few days
either researching, poking around the kernel headers, or experimenting
with different combinations.   As such, I've attached a .txt file
explaining the controlled (but probably not as thorough as you're used
to) benchmark I ran.   It's not a pretty graph, I know, but gimme a
break, I wrote it in vim and did the math with bc ;)

[-- Attachment #2: benchmark1.txt --]
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%%%%%%%%%%%%%
WITHOUT PATCH
%%%%%%%%%%%%%

dd if=/dev/zero of=/root/benchmark bs=512 count=20000
20000+0 records in
20000+0 records out
10240000 bytes (10 MB) copied, 0.674347 s, 15.2 MB/s

dd if=/dev/zero of=/root/benchmark bs=1024 count=20000
20000+0 records in
20000+0 records out
20480000 bytes (20 MB) copied, 0.89386 s, 22.9 MB/s

dd if=/dev/zero of=/root/benchmark bs=2048 count=20000
20000+0 records in
20000+0 records out
40960000 bytes (41 MB) copied, 1.36237 s, 30.1 MB/s

dd if=/dev/zero of=/root/benchmark bs=4096 count=20000
20000+0 records in
20000+0 records out
81920000 bytes (82 MB) copied, 2.81037 s, 29.1 MB/s

===============20000 blocks written averaged 24.325 MB/s
========================================================

dd if=/dev/zero of=/root/benchmark bs=512 count=40000
40000+0 records in
40000+0 records out
20480000 bytes (20 MB) copied, 1.31354 s, 15.6 MB/s

dd if=/dev/zero of=/root/benchmark bs=1024 count=40000
40000+0 records in
40000+0 records out
40960000 bytes (41 MB) copied, 1.8173 s, 22.5 MB/s

dd if=/dev/zero of=/root/benchmark bs=2048 count=40000
40000+0 records in
40000+0 records out
81920000 bytes (82 MB) copied, 3.23683 s, 25.3 MB/s

dd if=/dev/zero of=/root/benchmark bs=4096 count=40000
40000+0 records in
40000+0 records out
163840000 bytes (164 MB) copied, 6.79296 s, 24.1 MB/s

================== 40000 blocks written averaged 21.875 MB/s
============================================================

dd if=/dev/zero of=/root/benchmark bs=512 count=80000
80000+0 records in
80000+0 records out
40960000 bytes (41 MB) copied, 2.70969 s, 15.1 MB/s

dd if=/dev/zero of=/root/benchmark bs=1024 count=80000
80000+0 records in
80000+0 records out
81920000 bytes (82 MB) copied, 4.25879 s, 19.2 MB/s

dd if=/dev/zero of=/root/benchmark bs=2048 count=80000
80000+0 records in
80000+0 records out
163840000 bytes (164 MB) copied, 7.28753 s, 22.5 MB/s

dd if=/dev/zero of=/root/benchmark bs=4096 count=80000
80000+0 records in
80000+0 records out
327680000 bytes (328 MB) copied, 13.5436 s, 24.2 MB/s

==================== 80000 blocks written averaged 22.75 MB/s
=============================================================

%%%%%%%%%%
WITH PATCH (!)
%%%%%%%%%%

dd if=/dev/zero of=/root/benchmark bs=512 count=20000
20000+0 records in
20000+0 records out
10240000 bytes (10 MB) copied, 0.354359 s, 28.9 MB/s

dd if=/dev/zero of=/root/benchmark bs=1024 count=20000
20000+0 records in
20000+0 records out
20480000 bytes (20 MB) copied, 0.474818 s, 43.1 MB/s

dd if=/dev/zero of=/root/benchmark bs=2048 count=20000
20000+0 records in
20000+0 records out
40960000 bytes (41 MB) copied, 0.790466 s, 51.8 MB/s

dd if=/dev/zero of=/root/benchmark bs=4096 count=20000
20000+0 records in
20000+0 records out
81920000 bytes (82 MB) copied, 1.51956 s, 53.9 MB/s

================= 40000 blocks written averaged 44.425 MB/s (+82.6%)
====================================================================

dd if=/dev/zero of=/root/benchmark bs=512 count=40000
40000+0 records in
40000+0 records out
20480000 bytes (20 MB) copied, 0.731345 s, 28.0 MB/s

dd if=/dev/zero of=/root/benchmark bs=1024 count=40000
40000+0 records in
40000+0 records out
40960000 bytes (41 MB) copied, 1.06329 s, 38.5 MB/s

dd if=/dev/zero of=/root/benchmark bs=2048 count=40000
40000+0 records in
40000+0 records out
81920000 bytes (82 MB) copied, 1.85218 s, 44.2 MB/s

dd if=/dev/zero of=/root/benchmark bs=4096 count=40000
40000+0 records in
40000+0 records out
163840000 bytes (164 MB) copied, 4.08386 s, 40.1 MB/s

================= 40000 blocks written averaged 37.7 MB/s (+72.3%)
==================================================================

dd if=/dev/zero of=/root/benchmark bs=512 count=80000
80000+0 records in
80000+0 records out
40960000 bytes (41 MB) copied, 1.59573 s, 25.7 MB/s

dd if=/dev/zero of=/root/benchmark bs=1024 count=80000
80000+0 records in
80000+0 records out
81920000 bytes (82 MB) copied, 2.51223 s, 32.6 MB/s

dd if=/dev/zero of=/root/benchmark bs=2048 count=80000
80000+0 records in
80000+0 records out
163840000 bytes (164 MB) copied, 4.59659 s, 35.6 MB/s

dd if=/dev/zero of=/root/benchmark bs=4096 count=80000
80000+0 records in
80000+0 records out
327680000 bytes (328 MB) copied, 10.3018 s, 31.8 MB/s
(31.425-22.75)/22.75

=================== 80000 blocks written averaged 31.425 MB/s (+38.1%)
=====================================================================