Ciro Santilli @cirosantilli 37

This example is the same as nodejs/sequelize/raw/parallel_select_and_update.js, but going through Sequelize rather than with Sequelize raw queries. NONE is not supported for now to not have a transaction at all because lazy.

The examples illustrates: stackoverflow.com/questions/55452441/for-share-and-for-update-statements-in-sequelize

Sample invocation:

node --unhandled-rejections=strict ./parallel_select_and_update.js p 10 100 READ_COMMITTED UPDATE

where:

READ_COMMITTED: one of the keys documented at: sequelize.org/master/class/lib/transaction.js~Transaction.html which correspond to the standard sQL isolation levels. It not given, don't set one, defaulting to the database's/sequelize's default level.
UPDATE: one of the keys documented at: sequelize.org/master/class/lib/transaction.js~Transaction.html#static-get-LOCK. Update generates a SELECT FOR UPDATE in PostgreSQL for example. If not given, don't use any FOR xxx explicit locking.

Other examples:

node --unhandled-rejections=strict ./parallel_select_and_update.js p 10 100 READ_COMMITTED UPDATE

Then, the outcome is exactly as described at: nodejs/sequelize/raw/parallel_select_and_update.js:

READ_COMMITTED: fails
READ_COMMITTED UPDATE: works
REPEATABLE_READ: works, but is a bit slower, as it does rollbacks
This case also illustrates Sequelize transaction retries, since in this transaction isolation level transactions may fail:

 Read the full article

nodejs/sequelize/raw/parallel_select_and_update_deterministic.js Updated 2025-07-16

 View more

This example contains a deterministic demo of when postgreSQL serialization failures may happen.

Tested on PostgreSQL 13.5.

 Read the full article

nodejs/sequelize/raw/parallel_select_and_update.js Updated 2025-07-16

 View more

This example is similar to nodejs/sequelize/raw/parallel_update_async.js, but now we are doing a separate SELECT, later followed by an update:

SELECT FROM to get i
update on Js code newI = i + 1
UPDATE SET the newI

Although this specific example is useless in itself, as we could just use UPDATE "MyInt" SET i = i + 1 as in nodejs/sequelize/raw/parallel_update_async.js, which automatically solves any concurrency issue, this kind of code could be required for example if the update was a complex function not suitably implemented in SQL, or if the update depends on some external data source.

Sample execution:

node --unhandled-rejections=strict ./parallel_select_and_update.js p 2 10 'READ COMMITTED'

which does:

PostgreSQL, see other databases options at SQL example
2 threads
10 increments on each thread

Another one:

node --unhandled-rejections=strict ./parallel_select_and_update.js p 2 10 'READ COMMITTED' 'FOR UPDATE'

this will run SELECT FOR UPDATE rather than just SELECT

Observed behaviour under different SQL transaction isolation levels:

READ COMMITTED: fails. Nothing in this case prevents:
- thread 1: SELECT, obtains i = 0
- thread 2: SELECT, obtains i = 0
- thread 2: newI = 1
- thread 2: UPDATE i = 1
- thread 1: newI = 1
- thread 1: UPDATE i = 1
REPEATABLE READ: works. the manual mentions that if multiple concurrent updates would happen, only the first commit succeeds, and the following ones fail and rollback and retry, therefore preventing the loss of an update.
READ COMMITTED + SELECT FOR UPDATE: works. And does not do rollbacks, which probably makes it faster. With p 10 100, REPEATABLE READ was about 4.2s and READ COMMITTED + SELECT FOR UPDATE 3.2s on Lenovo ThinkPad P51 (2017).
SELECT FOR UPDATE should be enough as mentioned at: www.postgresql.org/docs/13/explicit-locking.html#LOCKING-ROWS
FOR UPDATE causes the rows retrieved by the SELECT statement to be locked as though for update. This prevents them from being locked, modified or deleted by other transactions until the current transaction ends. That is, other transactions that attempt UPDATE, DELETE, SELECT FOR UPDATE, SELECT FOR NO KEY UPDATE, SELECT FOR SHARE or SELECT FOR KEY SHARE of these rows will be blocked until the current transaction ends; conversely, SELECT FOR UPDATE will wait for a concurrent transaction that has run any of those commands on the same row, and will then lock and return the updated row (or no row, if the row was deleted). Within a REPEATABLE READ or SERIALIZABLE transaction, however, an error will be thrown if a row to be locked has changed since the transaction started. For further discussion see Section 13.4.

A non-raw version of this example can be seen at: nodejs/sequelize/parallel_select_and_update.js.

 Read the full article

Gibbs free energy Updated 2025-07-16

 View more

TODO understand more intuitively how that determines if a reaction happens or not.

Δ G = Δ H - T Δ S

(1)

At least from the formula we see that:

the more exothermic, the more likely it is to occur
if the entropy increases, the higher the temperature, the more likely it is to occur
- otherwise, the lower the temperature the more likely it is to occur

A prototypical example of reaction that is exothermic but does not happen at any temperature is combustion.

Video 1.

Lab 7 - Gibbs Free Energy by MJ Billman (2020)

Source. Shows the shift of equilibrium due to temperature change with a color change in a HCl CoCl reaction. Unfortunately there are no conclusions because its student's homework.

 Read the full article

nodejs/sequelize/raw/trigger_count.js Updated 2025-07-16

 View more

In this example we cache track the number of posts per user on a cache column.

 Read the full article

numpy/fft.py Updated 2025-07-16

 View more

Output:

sin(t)
fft
real 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10
rfft
real 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 0 0 0 0 0 0 0

sin(t) + sin(4t)
fft
real 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 -10 0 0 0 0 0 0 0 0 0 0 0 10 0 0 10
rfft
real 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 -10 0 0 0 0 0 0

With our understanding of the discrete Fourier transform we see clearly that:

the signal is being decomposed into sinusoidal components
because we are doing the Discrete Fourier transform of a real signal, for the fft, $X_{k} = \overset{ˉ}{X_{N - k}}$ so there is redundancy in the. We also understand that rfft simply cuts off and only keeps half of the coefficients

 Read the full article

python/getitem_complex.py Updated 2025-07-16

 View more

 Read the full article

python/pytorch/matmul.py Updated 2025-07-16

 View more

Matrix multiplication example.

Fundamental since deep learning is mostly matrix multiplication.

NumPy does not automatically use the GPU for it: stackoverflow.com/questions/49605231/does-numpy-automatically-detect-and-use-gpu, and PyTorch is one of the most notable compatible implementations, as it uses the same memory structure as NumPy arrays.

Sample runs on P51 to observe the GPU speedup:

$ time ./matmul.py g 10000 1000 10000 100
real    0m22.980s
user    0m22.679s
sys     0m1.129s
$ time ./matmul.py c 10000 1000 10000 100
real    1m9.924s
user    4m16.213s
sys     0m17.293s

 Read the full article

python/sphinx Updated 2025-07-16

 View more

To run each example and see the output run:

./build.sh
xdg-open out/index.html

 Read the full article

python/sphinx/class Updated 2025-07-16

 View more

Basic class example.

 Read the full article

python/sphinx/union Updated 2025-07-16

 View more

stackoverflow.com/questions/34647966/how-to-express-multiple-types-for-a-single-parameter-or-a-return-value-in-docstr/40801906#40801906

 Read the full article

react/ref-click-counter.html Updated 2025-07-16

 View more

Dummy example of using a React ref This example is useless and to the end user seems functionally equivalent to react/hello.html.

It does however serve as a good example of what react does that is useful: it provides a "clear" separation between state and render code (which becomes once again much less clear in React function components.

Notably, this example is insane because at:

<button onClick={() => {
  elem.innerHTML = (parseInt(elem.innerHTML) + 1).toString()

we are extracing state from some random HTML string rather than having a clean JavaScript variable containing that value.

In this case we managed to get away with it, but this is in general not easy/possible.

 Read the full article

riscv/timer.S Updated 2025-07-16

 View more

TODO: the interrupt is firing only once:

www.reddit.com/r/RISCV/comments/ov4vhh/timer_interrupt/

Adapted from: danielmangum.com/posts/risc-v-bytes-timer-interrupts/

Tested on Ubuntu 23.10:

sudo apt install binutils-riscv64-unknown-elf qemu-system-misc gdb-multiarch
cd riscv
make

Then on shell 1:

qemu-system-riscv64 -machine virt -cpu rv64 -smp 1 -s -S -nographic -bios none -kernel timer.elf

and on shell 2:

gdb-multiarch timer.elf -nh -ex "target remote :1234" -ex 'display /i $pc' -ex 'break *mtrap' -ex 'display *0x2004000' -ex 'display *0x200BFF8'

GDB should break infinitel many times on mtrap as interrupts happen.

 Read the full article

test_executables.js Updated 2025-07-16

 View more

This script tests all executables under a selected directory.

Ciro Santilli has been writing scripts of that type for a long time in order to test his programming self-learning setups with asserts.

The most advanced of those being the test system of Linux Kernel Module Cheat.

But had too much stuff that would be specific to that project, so Ciro decided to start this new one in Node.js, hopefully it will also be the last he ever writes.

A sample usage of the test library can be seen at: nodejs/sequelize/test.

 Read the full article

 There are unlisted articles, also show them or only show them.