On Thu, 21 Jan 2010, Stéphane Ducasse wrote:
Hi levente
I was wondering why the bufferedFileStream made the system faster.
I mean are querying the same contents on several occasion?
As Nicolas said, not all primitives have the same cost. The file
primitives cost a lot more than primitive 65 or some smalltalk code.
And as Igor said accessing the disk costs a lot more than accessing
memory, but that only counts in rare cases.
Here's an example showing that the file primitive costs almost the same if
it reads 1 or 2000 bytes:
#(1 2000) collect: [ :bufferSize |
(1 to: 5) collect: [ :run |
StandardFileStream readOnlyFileNamed: SourceFiles second name
do: [ :file |
| buffer fileID |
fileID := file instVarNamed: #fileID.
buffer := String new: bufferSize.
Smalltalk garbageCollect.
[ 1 to: 100000 do: [ :each |
file
primSetPosition: fileID to: 0;
primRead: fileID into: buffer
startingAt: 1 count: bufferSize ] ] timeToRun ] ] ]
===> #(
#(458 466 463 466 467)
#(482 484 492 483 484))
This means that a primitive send takes ~2.5 microseconds so it can be used
~400000 times/second.
If you read one byte/primitive that means 400KB/sec, if you read 2000
bytes/primitive you get 800MB/sec (your disk may be slower of course).
Using the buffer you get the following values:
(1 to: 10) collect: [ :run |
StandardFileStream readOnlyFileNamed: SourceFiles second name do: [
:file |
Smalltalk garbageCollect.
[ 1 to: 100000 do: [ :each |
file next ] ] timeToRun ] ]
===> #(15 14 12 11 12 12 11 12 12 12)
It takes ~12 milliseconds to read 100KB if you read the bytes
one-by-one. That means 8.3MB/sec. This is far from 800MB/sec, but much
better than the 400KB/sec.
Why does it matter? Because MultiByteFileStream (which is the default
FileStream) can only read (and convert) single bytes.
Levente
Stef
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