Memory required for struct array

2023 8 22
1 个回答
更新时间：2023 8 22
6 次查看（30 天）

0 个投票

In this answer https://www.mathworks.com/matlabcentral/answers/2010767-struct-function-in-matlab
Walter Roberson gives some number about memory used by struct
it seems to me the numbers derived by Walter are not correct.
In this thread I attempt to fix, mostly by reverse engineering. Note that this are not documented and possibly depend . on the MATLAB version. Currently it's R2023a.
So there is no question explicitly formulated here

请先登录，再回答此问题。

回答（1 个）

Bruno Luong
Bruno Luong 2023-8-22
编辑：Bruno Luong 2023-8-22
Ran in:

0 个投票

Ran in:
The function structbase_bytesize_fun that computes the "base" memory of the struct s is as following:
fieldname_bytesize = namelengthmax() + 1; % == 64
pointer_bytesize = 8;
mxArray_byteSize = 96;
structbase_bytesize_fun = @(s) length(fieldnames(s)) * ...
(fieldname_bytesize + pointer_bytesize*numel(s));
When the struct array is allocated without specify meaningful field values, the field values are affect to [] and no more memory is allocated.
s=struct('f1',cell(1,10),'f2',cell(1,10))
s = 1×10 struct array with fields:
f1 f2
whos s
Name Size Bytes Class Attributes s 1x10 288 struct
structbase_bytesize_fun(s)
ans = 288
When a field of a structure is set by a rhs the byte size of the rhs must be added
rhs = (1:3);
rhsinfo = whos('rhs');
s(end).f1 = rhs;
whos s
Name Size Bytes Class Attributes s 1x10 408 struct
struct_bytesize = structbase_bytesize_fun(s) + rhsinfo.bytes + mxArray_byteSize
struct_bytesize = 408
rhs2 = 1:4;
rhs2info = whos('rhs2');
[s(1:5).f2] = deal(rhs2);
whos s
Name Size Bytes Class Attributes s 1x10 1048 struct
struct_bytesize = structbase_bytesize_fun(s) + ...
1*(rhsinfo.bytes + mxArray_byteSize) + ... % s(end).f1
5*(rhs2info.bytes + mxArray_byteSize) % s(1:5).f2]
struct_bytesize = 1048
NOTE that on top of that memory seen by whos command, one need to count the memory taken header of s itself, which is hide by whos, meaning 96 bytes (mxArray_byteSize) needs to be added on top of that struct_bytesize.
It is not clear to me if shared data is correctly count (reduced memory), for example
[s(1:5).f2] = deal(rhs2);
would s(1:5).f2 share one copy of the rhs somewhere?

4 个评论
显示 2更早的评论 隐藏 2更早的评论

Bruno Luong
Bruno Luong 2023-8-22
Of course once you have the structure built, it's better to use whos to get the memory occupied.
The formula is useful to predict memory requirement without building the structure.
Walter Roberson
Walter Roberson 2023-8-22
Ran in:
Ran in:
clear s
s(100).foo = [];
whos s
Name Size Bytes Class Attributes s 1x100 960 struct
clear s
s(99).foo = [];
whos s
Name Size Bytes Class Attributes s 1x99 952 struct
960 - 952
ans = 8
so the number of extra bytes for each numel() must be 8, not 16.
Bruno Luong
Bruno Luong 2023-8-22
OK actually the number of extra bytes is
8 * (numberoffields * numel(s))
The fields data pointers are 2D array.
I fix my code.
Walter Roberson
Walter Roberson 2023-8-22
At the moment, I do not immediately see a difference between your size predictions and my previous enumeration?
My discussion was slightly sloppy in taking about sizes for additional entries, under the assumption that at least one entry was assigned to for each fieldname; it probably would have been better to rewrite them to a base of no entries written to. On the other hand, when no entries at all are allocated to a field, it looks like the 8-bytes-per-element pointer table is not allocated at all.

请先登录，再进行评论。

类别

在 帮助中心 和 File Exchange 中查找有关 Structures 的更多信息

产品

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by