On 4/19/23 19:51, Jessica Clarke wrote:
On 19 Apr 2023, at 18:24, Hans Petter Selasky <hsela...@freebsd.org> wrote:
On 4/19/23 17:46, Brooks Davis wrote:
On Wed, Apr 19, 2023 at 12:06:26PM +0000, Hans Petter Selasky wrote:
The branch main has been updated by hselasky:
URL:
https://cgit.FreeBSD.org/src/commit/?id=8dcf3a82c54cb216df3213a013047907636a01da
commit 8dcf3a82c54cb216df3213a013047907636a01da
Author: Hans Petter Selasky <hsela...@freebsd.org>
AuthorDate: 2022-09-08 10:16:43 +0000
Commit: Hans Petter Selasky <hsela...@freebsd.org>
CommitDate: 2023-04-19 12:04:22 +0000
libc: Implement bsort(3) a bitonic type of sorting algorithm.
The bsort(3) algorithm works by swapping objects, similarly to
qsort(3),
and does not require any significant amount of additional memory.
The bsort(3) algorithm doesn't suffer from the processing time issues
known the plague the qsort(3) family of algorithms, and is bounded by
a complexity of O(log2(N) * log2(N) * N), where N is the number of
elements in the sorting array. The additional complexity compared to
mergesort(3) is a fair tradeoff in situations where no memory may
be allocated.
The bsort(3) APIs are identical to those of qsort(3), allowing for
easy drop-in and testing.
The design of the bsort(3) algorithm allows for future parallell CPU
execution when sorting arrays. The current version of the bsort(3)
algorithm is single threaded. This is possible because fixed areas
of the sorting data is compared at a time, and can easily be divided
among different CPU's to sort large arrays faster.
Reviewed by: gbe@, delphij@, pauamma_gundo.com (manpages)
Sponsored by: NVIDIA Networking
Differential Revision: https://reviews.freebsd.org/D36493
Why was this needed? I'm not really a fan to adding new, non-standard
sorts without a clear use case. I understand it has some specific
advantages vs qsort, but are we going to use it? Doing so will make our
code less portable so we almost certainly can't s/qsort/bsort/.
Hi Brooks,
My long term plan is to get bsort() to replace qsort(), but because the two
algorithms have different characteristics, then some people may complain it is
good for me, but not for you. I want there to be an option besides from
qsort(), which does not use malloc() as an integral part of sorting. And is
faster than O(N*N) sorting (still the worst case for qsort in FreeBSD).
Why not do an adaptive qsort instead like other standard libraries?
Also, nothing actually says qsort doesn’t allocate memory; in fact,
glibc’s own documentation states that one should not assume it is
in-place and doesn’t allocate.
Hi Jessica,
qsort() is frequently used to do all kinds of sorting, and some pointed out
that qsort() can technically be any kind of sorting algorithm, but looking
around it is not.
Because there are variants that are guaranteed n log n? Which is better
than your n log^2 n.
Only so-called mergesort() algorithms can do sorting in O(N log N) time,
from what I know. And it needs to allocate working memory, when the
sorting arrays get large. From past experience, malloc() is a problem in
the fast path. If only mergesort() could pre-allocate that memory, or
have a pointer to pass working memory. What is the point of allocating
and freeing memory over and over again, in certain applications, doing
frequent sorting. The API is broken.
When I build applications of my own, I always use mergesort(), which depend on
malloc(). Having a dependency on a certain memory allocator to get performance
is not good.
I want to distinguish from qsort() by calling it bsort(). If people use bsort()
they know what they get cross-platform.
No they don’t, bsort can mean multiple things.
In the C-code standard domain, can you give examples of different
meanings of bsort() which are established? I'm not aware of any such
existing usage.
If you want a specific
algorithm, put it in your program, but please don’t, just use a
standardised function like qsort.
Sorting is a quite generic thing.
Likewise with <sys/queue.h> we support four different ways to make
lists. And then after epoch(9) was introduced came another three ways,
basically replicas of <sys/queue.h> with different properties (See:
sys/contrib/ck/include/ck_queue.h)
Personally I think the same about mergesort(), qsort() and bsort(). They
are so different and unique ways to sort that we should have all of them
in libc. They each serve a purpose.
If people use qsort() the output is random basically. It helps very little my
application works on FreeBSD, but not Linux, for example.
No, the output is sorted, which is far from random. And if you need a
stable sort you should ask for one.
Let me rephrase: Random with regards to execution time.
mergesort() : not for realtime applications (syscalls can sleep when
getting working memory)
qsort() : usually fast, but not always
bsort() : Worst case is "N * log2 N" compared to "N * N" (qsort), and no
syscalls are involved.
In FreeBSD qsort() is typically used for sorting files up to 16K entries per
directory. Even if qsort() can explode in time, probably it's not that
important. But when using qsort() for sorting millions of mathematical
expressions, for example, doing number analysis, this is unacceptable.
I think "C.A.R. Hoare's quicksort" technique is designed for single CPU systemsf only.
Even if the best-case average is "N*log2(N)", that amount of processing cannot be split
by multiple CPUs. The algorithm is serial as such.
The bsort() algorithm is much more NCPU friendly, because it can split the work
into fixed size smaller and independent work loads. Otherwise the work load
doubles every time you merge two sorted lists. And when the number of lists to
merge is fewer than the number of CPUs available, your system runs out of guts
basically.
I highly doubt you want a libc sort routine to start spawning threads.
Right. I'm thinking more like Grand Dispatch Central, though not that
common in FreeBSD, we still have a wiki page:
https://wiki.freebsd.org/GrandCentralDispatch
bsort() itself doesn't have to create threads to be SMP'ed, but can
provide the basic functions for SMP sorting.
When sorting records, the clever way to do it, is to make a list of
pointers to those records, and sort based on that, swapping pointers
first, then records in the end.
Technically you could have 4 CPUs go in a tight spin on different parts
of the bsort() algorithm, and by use of existing atomic compare and set
pointer operations, you could make a fully deterministic sorting
algorithm, even if the different CPUs clobber on the same memory locations.
This is something I want to explore later.
Neither mergesort() nor qsort() can do that.
That also seems extremely contradictory to your claim about wanting one
that doesn’t allocate memory, but creating threads is going to do much
more than just that, and would scream POLA violation to me.
See my explanation above. TAILQ_INSERT_TAIL() can also be used in
multithreading environments, without having to link to pthreads.
I also note that the swap code is pointlessly slow for size > 256 and
should almost certainly use aliasing with matching words like memcpy
implementations do. Doing so would make it easier to port this code to
CHERI where that is required.
I totally agree about the swap code being pointless. And I tried to look where
is memswap(), but it was not there. This kind of swapping is done many places,
and we could benefit from having a compiler supported memswap() function. What
do you think?
We agree: https://github.com/CTSRD-CHERI/cheribsd/issues/207. But the
world doesn’t have that in C.
Yes, that's a good idea! I'll try to follow that lead.
We don’t need new non-standard sorting routines in libc.
We don't need English tea either ;-)
--HPS
