Geek Answers Handbook

JavaScript test on V8

function add(x, y) { return x + y; } console.time("time1"); for (var i = 0; i < 90000000; ++i) { add(1, 2); add('a','b'); } console.timeEnd("time1"); function addText(x, y) { return x + y; } function addNumber(x, y) { return x + y; } console.time("time2"); for (var i = 0; i < 90000000; ++i) { addNumber(1, 2); addText('a','b'); } console.timeEnd("time2");

result: time1: 1481ms, time2: 102ms,

I run this test on nodejs, but I don't know why the first test run is slower than the second.

+8
javascript
source share
2 answers

This is due to deoptimization occurring under the hood due to a change in parameter type. To know this, we need to know what V8 optimization is:

Embedding

This is an important optimization and is critical to improving productivity. It replaces a function call with a function case to speed up execution. If the function is small in size, it will be integrated. This assumes that parameters are fixed in function calls.

Deoptimisation

V8 does optimization under certain assumptions. If this is not correct, he must undo these optimizations so that he can run the code correctly. The experience is bad because it takes time, and the replaced code is less efficient.

To see this in action, run the code with the parameters --trace_opt --trace_opt_stats --trace_deopt --trace-inlining

 [deoptimize context: cbf30f14679] [marking addText 0xcbf30fc5ca0 for recompilation, reason: small function, ICs with typeinfo: 1/1 (100%)] [optimizing: addText / cbf30fc5ca1 - took 0.058, 0.092, 0.000 ms] Compiled: 1 functions with 42 byte source size in 0.150000ms. [marking addNumber 0xcbf30fc5ce8 for recompilation, reason: small function, ICs with typeinfo: 1/1 (100%)] [optimizing: addNumber / cbf30fc5ce9 - took 0.019, 0.033, 0.000 ms] Compiled: 2 functions with 84 byte source size in 0.202000ms. [marking 0x2c9408e1b3d0 for recompilation, reason: not much type info but very hot, ICs with typeinfo: 1/12 (8%)] Inlined addNumber called from . Inlined addText called from . [optimizing: / 2c9408e1b3d1 - took 0.131, 0.229, 0.000 ms] Compiled: 3 functions with 898 byte source size in 0.562000ms. time1: 126ms **** DEOPT: at bailout #11, address 0x0, frame size 192 [deoptimizing: begin 0x2c9408e1b3d1 @11] translating => node=111, height=32 0x7fff72081080: [top + 104] <- 0x2c9408e1b4e9 ; [sp + 32] 0x2c9408e1b4e9 <an Object> 0x7fff72081078: [top + 96] <- 0x58df1704121 <undefined> ; literal 0x7fff72081070: [top + 88] <- 0x58df1704121 <undefined> ; literal 0x7fff72081068: [top + 80] <- 0x58df1704121 <undefined> ; literal 0x7fff72081060: [top + 72] <- 0x58df1704121 <undefined> ; literal 0x7fff72081058: [top + 64] <- 0x58df1704121 <undefined> ; literal 0x7fff72081050: [top + 56] <- 0x23522b3122be ; caller pc 0x7fff72081048: [top + 48] <- 0x7fff720810b0 ; caller fp 0x7fff72081040: [top + 40] <- 0x2c9408e1b0d9; context 0x7fff72081038: [top + 32] <- 0x2c9408e1b3d1; function 0x7fff72081030: [top + 24] <- 0x2c9408e1b389 ; [sp + 40] 0x2c9408e1b389 <JS Function add> 0x7fff72081028: [top + 16] <- 0x2c9408e1b2f9 ; [sp + 48] 0x2c9408e1b2f9 <JS Function addText> 0x7fff72081020: [top + 8] <- 0x2c9408e1b341 ; [sp + 56] 0x2c9408e1b341 <JS Function addNumber> 0x7fff72081018: [top + 0] <- 0 ; rbx (smi) [deoptimizing: end 0x2c9408e1b3d1 => node=111, pc=0x23522b364aa0, state=NO_REGISTERS, alignment=no padding, took 0.076 ms] [removing optimized code for: ] [marking add 0x2c9408e1b388 for recompilation, reason: small function, ICs with typeinfo: 1/1 (100%)] [optimizing: add / 2c9408e1b389 - took 0.013, 0.024, 0.000 ms] Compiled: 4 functions with 939 byte source size in 0.599000ms. [marking 0x2c9408e1b3d0 for recompilation, reason: hot and stable, ICs with typeinfo: 2/12 (16%)] Inlined addNumber called from . Inlined addText called from . Inlined add called from . Inlined add called from . [optimizing: / 2c9408e1b3d1 - took 0.100, 0.194, 0.000 ms] Compiled: 5 functions with 1753 byte source size in 0.893000ms. time2: 1759ms

The first cycle is deoptimized due to a change in the parameter and lasts longer. This cannot be explained by deleting / skipping the cache because there is no read / write.

Source: http://floitsch.blogspot.in/2012/03/optimizing-for-v8-inlining.html

+4
source share

I think this is about cache. in the 1st, the javascript function dont know ' add ' do something. but in the second javascript they understand that "addNumber" returns the sum of x and y and " addText " return x+''+y

+1
source share

More articles:


All Articles