Photo cross-post

Sunday, 15 June 2025 06:02 am

Photo cross-post

Sunday, 15 June 2025 05:15 am
andrewducker: (Default)
[personal profile] andrewducker


It was about ten seconds later that we realised how terrible Crocs are for climbing.
Original is here on Pixelfed.scot.

Confused by Disney ineptitude

Saturday, 14 June 2025 12:03 pm
andrewducker: (Default)
[personal profile] andrewducker
Two weeks after seeing the CGI Lilo and Stitch at the cinema I'm watching the original with the kids. And it's so much better. The direction, the writing, the acting are all just much higher quality.

The remake felt much clumsier. And I don't really understand why.

Edit: Just realised that they entirely cut the Ugly Duckling part from the remake. Why would you do that? It's key to Stitch's arc!
And all of the bits where Lilo how to be like Elvis.

In fact, nearly all of the bits where Lilo talked to Stitch and built a relationship with him.

In (near) Prague

Saturday, 14 June 2025 08:18 am
rmc28: Rachel in hockey gear on the frozen fen at Upware, near Cambridge (Default)
[personal profile] rmc28

On an ice hockey camp in Slaný, near Prague. I flew out on Thursday afternoon with two friends from Kodiaks. We arrived at the rink hotel in time to check in, have a little walk down to the nearby supermarket and get food, and settle in for the night. For reasons the three of us were all sharing a dormitory room the first night, and we decided the perfect film to watch over our picnic dinner was Inside Out 2 - also set at a 3-day hockey camp. I hadn't seen it before, though the other two had, and I enjoyed it very much.

Friday morning was pretty relaxed; a fourth Kodiak joined us after leaving home at awful-o-clock in the morning, and we were moved into the nicer ensuite twin rooms in pairs for the rest of the camp. We met in the dressing room at 1pm, were on ice at 2pm and again at 6pm, with a stickhandling session in between. Then dinner at 8 and falling into bed not long after.

It's excellent coaching, I'm being pushed well out of my comfort zone and the balance of drill and rest in each session and between sessions is just right. I hit my "cannot actually skate any more" limit about 3 minutes before the end of the last ice session.

Today will be two ice sessions at either end of the day, with video review (argh), optional swim+spa (yes!), and stickhandling again in between. My muscles this morning are making themselves known but I'm not exhausted. All is good. Time to go get changed.

andrewducker: (my brain)
[personal profile] andrewducker
Reading this article on advice to teachers in the UK about using AI, they suggest using it for things like "marking quizzes" and "generating routine letters".

And what really annoys me about this is that it's a perfect example of where simple automation could be used without the need for AI.

The precise example in the article is "Generate a letter to parents about a head lice outbreak." - which is a fairly common thing to happen in schools. So why on earth isn't there one standard letter per school, if not one standard letter for the whole country, that can be reused by absolutely everyone whenever this standard event happens? Why does this require AI to generate a new one every time, rather than just being a standard email that gets sent?

Same with marking quizzes. If children get multiple-choice quizzes regularly across all schools, and marking them uses precious teacher time, why is there not a standard piece of software, paid for once (or written once internally) which enables all children to do quizzes in a standard way, and get them marked automatically?

If we're investing a bunch of money into automating the various processes that teachers spend far too much time on, start with simple automation, which is cheap, easy, and reliable.

Also, wouldn't it be sensible to do some research into how accurately AI marks homework *before* you tell teachers to use it to do that? Here's some research from February which shows that its agreement with examiners was only 0.61 (where 1.00 would be perfect agreement). So I'm sceptical about the quality of the marking it's going to be doing...

performance of random floats (amended)

Monday, 9 June 2025 10:00 pm
fanf: (Default)
[personal profile] fanf

After I found some issues with my benchmark which invalidated my previous results, I have substantially revised my previous blog entry. There are two main differences:

  • A proper baseline revealed that my amd64 numbers were nonsense because I wasn’t fencing enough, and after tearing my hair out and eventually fixing that I found that the bithack conversion is one or two cycles faster.

  • A newer compiler can radically improve the multiply conversion on arm64 so it’s the same speed as the bithack conversion; I've added some source and assembly snippets to the blog post to highlight how nice arm64 is compared to amd64 for this task.

To-read pile, 2025, May

Monday, 9 June 2025 07:31 pm
rmc28: (reading)
[personal profile] rmc28

Books on pre-order:

  1. Queen Demon (Rising World 2) by Martha Wells (7 Oct 2025)

Books acquired in May:

  • and read:
    1. Copper Script by KJ Charles
    2. Red Boar's Baby by Lauren Esker
  • and unread:
    1. The Wrath & The Dawn by Renée Ahdieh [3]
    2. The Unexpected Inheritance of Inspector Chopra by Vaseem Khan [3]
    3. Kidnap on the California Comet by M.G. Leonard & Sam Sedgman [3]
    4. Betrayal (Trinity 1) by Fiona McIntosh [3]

Borrowed books read in May:

  1. The Good Thieves by Katherine Rundell
  2. One Christmas Wish by Katherine Rundell
  3. You Have a Match by Emma Lord [2][6]

I continue to not read much (by my standards). I did not manage to read any of the physical books I had out of the library until they needed to be returned, and I've got several half-finished books in progress. (Oh, and in writing this I've realised I already have the Renée Ahdieh book in ebook, and haven't read it there either!)

[1] Pre-order
[2] Audiobook
[3] Physical book
[4] Crowdfunding
[5] Goodbye read
[6] Cambridgeshire Reads/Listens
[7] FaRoFeb / FaRoCation / Bookmas / HRBC
[8] Prime Reading / Kindle Unlimited

EHRC nonsense

Monday, 9 June 2025 11:14 am
lnr: Halloween 2023 (Default)
[personal profile] lnr

We still haven't met with Senior Management: it's now due tomorrow, in person. I'm gently trying not to panic.

There's still been no message of support to all members of staff and students from the University, and nothing at all from the department. Though I understand they're still in discussions in the background. This is frustrating.

The subject was raised at a recent All Staff meeting (in which people submit questions as text, and senior management attempt to answer them). We were given broad assurances that the university values and supports trans people, but nothing actually useful or genuinely supportive was said.

In the meantime a new EHRC chair is due to be appointed, and they're considering a person with a known anti-trans background. There's an Open Letter available to sign in protest, written by a very good friend and colleague: https://docs.google.com/forms/d/e/1FAIpQLSe_Y77t7CQqKjdGifNa0lE3HKjDAb1UoJdjuLAbInhIQsRMhw/viewform

I've also seen a good template if you want to write directly: https://docs.google.com/document/d/1865KMfu24JgmwnWmYXaVc3jlzj5uQFEq69hXMxKP6BU/edit?tab=t.0

And I wrote my own version:

9th June 2025
Dear Women’s and Equalities Select Committee and Joint Committee on Human Rights,
Cc: Pippa Heylings, as my MP

I am writing to express my grave concern about the proposed appointment of Dr Mary-Ann Stephenson as the Chair of the Equality and Human Rights Commission.

I won't include a string of references here, because I think you will have seen them all already, but I think it is imperative that the next person appointed as Head of the EHRC should not be seen to have a strong anti-trans background. Trans people are currently scared. Scared for their jobs, if they cannot access their workplace in safety and dignity. Scared of being assaulted if they go to the "wrong" toilet. Scared of being outed as trans in public if they try to follow the new guidelines.

And I am scared as a cis woman, a woman who is not trans, at what is happening in our country, and what this means for my friends and colleagues and for trans people in general. For intersex people, non-binary people, and any woman who might be mistaken for being trans. Other women need to feel safe too, but excluding trans people is not the way to do this.

The EHRC needs to stand up for the rights of everyone, and to be seen to do so. I sincerely hope you will take this into account.

Kind Regards,

Eleanor Blair
Great Shelford, Cambridge, CB22

I'm not even going to attempt to get into the member of the EHRC who was quoted as effectively saying that trans people have been misled about their rights under the Equality Act for the last 15 years, and there will now be a period of adjustment, but they should just get used to having fewer rights than they thought they did. The Guardian changed their headline and reporting three times as a result of her protesting about being misquoted, but that seems to have been the gist of it. Not mentioning that the "misleading" guidance came from the EHRC themselves, and was based on the previous understanding of the Equalities Act and entirely consistent with it. FFS

performance of random floats

Sunday, 8 June 2025 03:15 am
fanf: (Default)
[personal profile] fanf

https://dotat.at/@/2025-06-08-floats.html

A couple of years ago I wrote about random floating point numbers. In that article I was mainly concerned about how neat the code is, and I didn't pay attention to its performance.

Recently, a comment from Oliver Hunt and a blog post from Alisa Sireneva prompted me to wonder if I made an unwarranted assumption. So I wrote a little benchmark, which you can find in pcg-dxsm.git.

(Note 2025-06-09: I've edited this post substantially after discovering some problems with the results.)

recap

Briefly, there are two basic ways to convert a random integer to a floating point number between 0.0 and 1.0:

  • Use bit fiddling to construct an integer whose format matches a float between 1.0 and 2.0; this is the same span as the result but with a simpler exponent. Bitcast the integer to a float and subtract 1.0 to get the result.

  • Shift the integer down to the same range as the mantissa, convert to float, then multiply by a scaling factor that reduces it to the desired range. This produces one more bit of randomness than the bithacking conversion.

(There are other less basic ways.)

code

The double precision code for the two kinds of conversion is below. (Single precision is very similar so I'll leave it out.)

It's mostly as I expect, but there are a couple of ARM instructions that surprised me.

bithack

The bithack function looks like:

double bithack52(uint64_t u) {
    u = ((uint64_t)(1023) << 52) | (u >> 12);
    return(bitcast(double, u) - 1.0);
}

It translates fairly directly to amd64 like this:

bithack52:
    shr     rdi, 12
    movabs  rax, 0x3ff0000000000000
    or      rax, rdi
    movq    xmm0, rax
    addsd   xmm0, qword ptr [rip + .number]
    ret
.number:
    .quad   0xbff0000000000000

On arm64 the shift-and-or becomes one bfxil instruction (which is a kind of bitfield move), and the constant -1.0 is encoded more briefly. Very neat!

bithack52:
    mov     x8, #0x3ff0000000000000
    fmov    d0, #-1.00000000
    bfxil   x8, x0, #12, #52
    fmov    d1, x8
    fadd    d0, d1, d0
    ret

multiply

The shift-convert-multiply function looks like this:

double multiply53(uint64_t u) {
    return ((double)(u >> 11) * 0x1.0p-53);
}

It translates directly to amd64 like this:

multiply53:
    shr       rdi, 11
    cvtsi2sd  xmm0, rdi
    mulsd     xmm0, qword ptr [rip + .number]
    ret
.number:
    .quad     0x3ca0000000000000

GCC and earlier versions of Clang produce the following arm64 code, which is similar though it requires more faff to get the constant into the right register.

multiply53:
    lsr     x8, x0, #11
    mov     x9, #0x3ca0000000000000
    ucvtf   d0, x8
    fmov    d1, x9
    fmul    d0, d0, d1
    ret

Recent versions of Clang produce this astonishingly brief two instruction translation: apparently you can convert fixed-point to floating point in one instruction, which gives us the power of two scale factor for free!

multiply53:
    lsr     x8, x0, #11
    ucvtf   d0, x8, #53
    ret

benchmark

My benchmark has 2 x 2 x 2 tests:

  • bithacking vs multiplying

  • 32 bit vs 64 bit

  • sequential integers vs random integers

I ran the benchmark on my Apple M1 Pro and my AMD Ryzen 7950X.

These functions are very small and work entirely in registers so it has been tricky to measure them properly.

To prevent the compiler from inlining and optimizing the benchmark loop to nothing, the functions are compiled in a separate translation unit from the test harness. This is not enough to get plausible measurements because the CPU overlaps successive iterations of the loop, so we also use fence instructions.

On arm64, a single ISB (instruction stream barrier) in the loop is enough to get reasonable measurements.

I have not found an equivalent of ISB on amd64, so I'm using MFENCE. It isn't effective unless I pass the argument and return values via pointers (because it's a memory fence) and place MFENCE instructions just before reading the argument and just after writing the result.

results

In the table below, the leftmost column is the number of random bits; "old" is arm64 with older clang, "arm" is newer clang, "amd" is gcc.

The first line is a baseline do-nothing function, showing the overheads of the benchmark loop, function call, load argument, store return, and fences.

The upper half measures sequential numbers, the bottom half is random numbers. The times are nanoseconds per operation.

         old    arm    amd

    00  21.44  21.41  21.42

    23  24.28  24.31  22.19
    24  25.24  24.31  22.94
    52  24.31  24.28  21.98
    53  25.32  24.35  22.25

    23  25.59  25.56  22.86
    24  26.55  25.55  23.03
    52  27.83  27.81  23.93
    53  28.57  27.84  25.01

The times vary a little from run to run but the difference in speed of the various loops is reasonably consistent.

The numbers on arm64 are reasonably plausible. The most notable thing is that the "old" multiply conversion is about 3 or 4 clock cycles slower, but with a newer compiler that can eliminate the multiply, it's the same speed as the bithacking conversion.

On amd64 the multiply conversion is about 1 or 2 clock cycles slower than the bithacking conversion.

conclusion

The folklore says that bithacking floats is faster than normal integer to float conversion, and my results generally agree with that, apart from on arm64 with a good compiler. It would be interesting to compare other CPUs to get a better idea of when the folklore is right or wrong -- or if any CPUs perform the other way round!

Photo cross-post

Saturday, 7 June 2025 12:29 pm
andrewducker: (Default)
[personal profile] andrewducker


My brother Mike got me this for my birthday, and it just takes a weight off my mind being able to say "bring the steam temperature up to 95 degrees and hold it there"

(Control over oil temperature when frying eggs is also awesome.)
Original is here on Pixelfed.scot.

A mostly-free day

Saturday, 7 June 2025 10:31 am
rmc28: Rachel post-game, slumped sideways in a chair eyes closed (tired)
[personal profile] rmc28

I'm playing an ice hockey game tonight in Cambridge, a charity fundraiser between Warbirds and Tri-Base Lightning. But until then I have a strangely unscheduled day. I might sleep or read or something.

I could post about what I've been up to lately!

Work:

  • spoke on a panel about effective 1:1s, it seemed to go well
  • played my usual Senior Tech Woman role for a colleague's recruitment panel, and am happy that our preferred candidate has apparently just accepted. (a frustrating number of timewasting applicants more or less obviously using LLMs to write their applications and generate their free-text statements on suitability for the role; I really resent having to wade through paragraphs of verbose buzzword bilge to ... fail to find any evidence they actually know how to do the job)

Hockey:

  • KODIAKS WON PLAYOFFS on the bank holiday weekend oh yes they did. So proud of the players, and definitely earned my share of reflected glory managing the team this season and running around half the weekend. League winners, Cup winners, Playoff winners, promotion to Division 1 next season, utter delight.
  • Very much an Insufficient Sleep weekend, we topped off the playoff win with a night out in Sheffield, I got back to my hotel as the sky was getting light, good times.
  • Kodiaks awards evening last night: lots of celebration of the hard work and lovely camaraderie of this group of players, A and B teams both. I got to announce and hand out the B team awards, and I received a really nice pair of gifts for me as manager: a canvas print of a post-final winners photo, and a personalised insulated travel mug (club logo and MANAGER on it). I love this team.
  • I'm still enjoying also playing with Warbirds, and have now been to a few summer Friday scrimmages run by Tri-Base. I went to a couple of Friday scrims at the end of last summer and felt everyone was very kind but I was pretty outclassed. I'm pleased to feel like I'm keeping up a bit better now after training a lot harder this last season.
  • I trained three days in a row this week (Warbirds Monday, Haringey Greyhounds tryouts in Alexandra Palace on Tuesday, Kodiaks Wednesday) and that was Too Much and I was pretty sore Wednesday evening and Thursday. Rest days are important even if I am much improved in fitness compared to this time last year.

Other:

  • I did a formal hall at my old College! Using my alumna rights and having a nice evening hanging out with old friends (who were the ones to suggest the plan). Good times, will do again but probably not this term.
  • I had an excessive number of books out from Suffolk libraries that needed returning, so I did a flying visit to Newmarket by bus last Saturday, this turned out to be the cheapest/quickest way across the county border. I managed to stick to my resolution not to borrow any more physical books but slipped and fell on the "withdrawn books for sale" stand. Managed to only come home with four.
  • I did a little indoor cricket the Friday before playoffs (it's now finished due to exam period), and some nets practice last Sunday, but I keep being too busy to actually play any of my team's games. I'd like to do more nets practice though, that was intense but also felt like I was beginning to improve.
  • I did a little table tennis with Active Staff but that's also now suspended for exams. I'm considering getting a cheap set of bats and balls for me and the family to go use at the local rec ground, or in the free indoor tables at the Grafton Centre.

Coming up: my summer is full of ice hockey camps and tournaments (Prague, Hull, Sheffield, Biarritz) and my old club Streatham have just announced all their summer training sessions will be "Summer Skills Camps" open to all interested WNIHL players, so I'm looking at going to London regularly again in July and August.

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