tail

[Barry]

> On 9 May 2022, at 17:41, ram at zedat.fu-berlin.de wrote:
> 
> Barry Scott <barry at barrys-emacs.org> writes:
>> Why use tiny chunks? You can read 4KiB as fast as 100 bytes
> 
>  When optimizing code, it helps to be aware of the orders of
>  magnitude

That is true and we’ll know to me, now show how what I said is wrong.

The os is going to DMA at least 4k, with read ahead more like 64k.
So I can get that into the python memory at the same scale of time as
1 byte because it’s the setup of the I/O that is expensive not the bytes
transferred.

Barry

> . Code that is more cache-friendly is faster, that is,
>  code that holds data in single region of memory and that uses
>  regular patterns of access. Chandler Carruth talked about this,
>  and I made some notes when watching the video of his talk:
> 
> CPUS HAVE A HIERARCHICAL CACHE SYSTEM
> (from a 2014 talk by Chandler Carruth)
> 
> One cycle on a 3 GHz processor            1   ns
> L1 cache reference                        0.5 ns
> Branch mispredict                         5   ns
> L2 cache reference                        7   ns          14x L1 cache
> Mutex lock/unlock                        25   ns
> Main memory reference                   100   ns          20xL2, 200xL1
> Compress 1K bytes with Snappy         3,000   ns
> Send 1K bytes over 1 Gbps network    10,000   ns  0.01 ms
> Read 4K randomly from SSD           150,000   ns  0.15 ms
> Read 1 MB sequentially from memory  250,000   ns  0.25 ms
> Round trip within same datacenter   500,000   ns   0.5 ms
> Read 1 MB sequentially From SSD   1,000,000   ns   1   ms 4x memory
> Disk seek                        10,000,000   ns  10   ms 20xdatacen. RT
> Read 1 MB sequentially from disk 20,000,000   ns  20   ms 80xmem.,20xSSD
> Send packet CA->Netherlands->CA 150,000,000   ns 150   ms
> 
>  . Remember how recently people here talked about how you cannot
>  copy text from a video? Then, how did I do it? Turns out, for my 
>  operating system, there's a screen OCR program! So I did this OCR 
>  and then manually corrected a few wrong characters, and was done!
> 
> 
> -- 
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>