New subject: libnestutil BlockVector Implementation

10 Aug 2021

Hi Itaru,

The problem we had with std::vector was how it allocates memory. When the memory allocated
for a vector is full, its capacity is increased by allocating a bigger chunk of memory
(commonly with double the capacity of the original vector), and all elements are moved to
the new allocation.

We observed that when connections are all put in one big vector, and the number of
connections in the vector becomes large, memory consumption from vector growth becomes
problematic. We therefore created a new container, BlockVector, to keep memory consumption
under control and avoid moving the elements when increasing the capacity. The BlockVector
achieves this by allocating an entire block of a fixed size every time the capacity has to
be increased. The BlockVector therefore becomes a vector of vectors, where the entire size
of each new block, which is a new vector in the vector of vectors, is allocated only once
when the block is created.

Some benchmarks were done during the review of the PR,

https://github.com/nest/nest-simulator/pull/1047

You can find the benchmarks here:

https://github.com/nest/nest-simulator/files/2505835/Bench_214_sq_db70dcc1e…

Note that BlockVector was called "Seque" at the time.

Best,
Håkon
________________________________
From: Itaru Kitayama &lt;itaru.kitayama(a)gmail.com&gt;
Sent: 10 August 2021 05:51
To: NEST User Mailing List &lt;users(a)nest-simulator.org&gt;
Subject: [NEST Users] libnestutil BlockVector Implementation

Hello Håkon,

As you implemented, an instance of BlockVector is a vector of vectors but
allows users to specify with one index. As I am having a few problems with
mapping BlockVetors to the device memory, I am simply wondering as to
why we can't just use std::vector< value_type >? If you guys observed
significant performance improvements with BlockVectors in the past,
would you mind pointing me to the papers or notes?

Thanks,
Itaru.
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Re: libnestutil BlockVector Implementation