问题 MATLAB：将几个变量保存到“-v7.3”（HDF5）。使用“-append”标志时，.mat文件似乎更快。怎么来的？

天啊。我可以重现。尝试了单附加变化;它甚至更快。看起来像“-append”只是神奇地使基于HDF5的save（）快30倍。我没有解释，但我想分享我发现的东西。

我将测试代码包装在一个函数中，重构它以使保存逻辑与测试数据结构无关，这样您就可以在其他数据集上运行它，并添加了一些更多的诊断输出。

到处都看不到大的加速。它在我的64位XP盒子和32位Server 2003盒子上是巨大的，在我的64位Windows 7盒子上很大，在32位XP盒子上不存在。 （虽然在Server 2003上有多个附加功能是一个巨大的损失。）在许多情况下R2010b速度较慢。也许HDF5附加或保存使用它只是摇滚新的Windows版本。 （XP x64实际上是Server 2003内核。）或者它可能只是一个机器配置差异。 XP x64机器上有一个快速的RAID，而32位XP的RAM比其余的少。你在运行什么操作系统和架构？你也可以试试这个复制品吗？

19:36:40.289: Testing speed, format=-v7.3, R2009b on PCWIN64, arch=AMD64, os=Microsoft(R) Windows(R) XP Professional x64 Edition 5.2.3790 Service Pack 2 Build 3790
19:36:55.930: Save the simple way:           11.493 sec
19:37:07.415: Save using multiple append:     1.594 sec
19:37:09.009: Save using one big append:      0.424 sec


19:39:21.681: Testing speed, format=-v7.3, R2009b on PCWIN, arch=x86, os=Microsoft Windows XP Professional 5.1.2600 Service Pack 3 Build 2600
19:39:37.493: Save the simple way:           10.881 sec
19:39:48.368: Save using multiple append:    10.187 sec
19:39:58.556: Save using one big append:     11.956 sec


19:44:33.410: Testing speed, format=-v7.3, R2009b on PCWIN64, arch=AMD64, os=Microsoft Windows 7 Professional  6.1.7600 N/A Build 7600
19:44:50.789: Save the simple way:           14.354 sec
19:45:05.156: Save using multiple append:     6.321 sec
19:45:11.474: Save using one big append:      2.143 sec


20:03:37.907: Testing speed, format=-v7.3, R2009b on PCWIN, arch=x86, os=Microsoft(R) Windows(R) Server 2003, Enterprise Edition 5.2.3790 Service Pack 2 Build 3790
20:03:58.532: Save the simple way:           19.730 sec
20:04:18.252: Save using multiple append:    77.897 sec
20:05:36.160: Save using one big append:      0.630 sec

这看起来很大。如果它支持其他数据集，我可能会在很多地方使用这个技巧。这也可能是MathWorks的一部分。他们是否可以在普通保存或其他操作系统版本中使用快速附加技术？

这是独立的repro功能。

function out = reproMatfileAppendSpeedup(nPasses, tests, imax, formats)
%REPROMATFILEAPPENDSPEEDUP Show how -append makes v7.3 saves much faster
%
% Examples:
% reproMatfileAppendSpeedup()
% reproMatfileAppendSpeedup(2, [], 0, {'7.3','7','6'}); % low-entropy test

if nargin < 1 || isempty(nPasses);  nPasses = 1;  end
if nargin < 2 || isempty(tests);    tests = {'basic','multiappend','bigappend'}; end
if nargin < 3 || isempty(imax);     imax = 255; end
if nargin < 4 || isempty(formats);  formats = '7.3'; end % -v7 and -v6 do not show the speedup
tests = cellstr(tests);
formats = cellstr(formats);

fprintf('%s: Testing speed, imax=%d, R%s on %s\n',...
    timestamp, imax, version('-release'), systemDescription());

tempDir = setupTempDir();
testData = generateTestData(imax);

testMap = struct('basic','saveSimple', 'multiappend','saveMultiAppend', 'bigappend','saveBigAppend');

for iFormat = 1:numel(formats)
    format = formats{iFormat};
    formatFlag = ['-v' format];
    %fprintf('%s: Format %s\n', timestamp, formatFlag);
    for iTest = 1:numel(tests)
        testName = tests{iTest};
        saveFcn = testMap.(testName);
        te = NaN(1, nPasses);
        for iPass = 1:nPasses
            fprintf('%s: %-30s', timestamp, [testName ' ' formatFlag ':']);
            t0 = tic;
            matFile = fullfile(tempDir, sprintf('converted-%s-%s-%d.mat', testName, format, i));
            feval(saveFcn, matFile, testData, formatFlag);
            te(iPass) = toc(t0);
            if iPass == nPasses
                fprintf('%7.3f sec      %5.3f GB used   %5.0f MB file   %5.3f sec mean\n',...
                    te(iPass), physicalMemoryUsed/(2^30), getfield(dir(matFile),'bytes')/(2^20), mean(te));
            else
                fprintf('%7.3f sec      %5.3f GB used\n', te(iPass), physicalMemoryUsed/(2^30));
            end
        end
        % Verify data to make sure we are sane
        gotBack = load(matFile);
        gotBack = rmfield(gotBack, intersect({'dummy'}, fieldnames(gotBack)));
        if ~isequal(gotBack, testData)
            fprintf('ERROR: Loaded data differs from original for %s %s\n', formatFlag, testName);
        end
    end
end

% Clean up
rmdir(tempDir, 's');

%%
function saveSimple(file, data, formatFlag)
save(file, '-struct', 'data', formatFlag);

%%
function out = physicalMemoryUsed()
if ~ispc
    out = NaN;
    return; % memory() only works on Windows
end
[u,s] = memory();
out = s.PhysicalMemory.Total - s.PhysicalMemory.Available;

%%
function saveBigAppend(file, data, formatFlag)
dummy = 0;
save(file, 'dummy', formatFlag);
fieldNames = fieldnames(data);
save(file, '-struct', 'data', fieldNames{:}, '-append', formatFlag);

%%
function saveMultiAppend(file, data, formatFlag)
fieldNames = fieldnames(data);
for i = 1:numel(fieldNames)
    if (i > 1); appendFlag = '-append'; else; appendFlag = ''; end
    save(file, '-struct', 'data', fieldNames{i}, appendFlag, formatFlag);
end


%%
function testData = generateTestData(imax)
nBlocks = 40;
blockSize = [65 480 240];
for i = 1:nBlocks
    testData.(sprintf('block_%03u', i)) = struct('blockNo',i,...
        'frames', randi([0 imax], blockSize, 'uint8'));
end

%%
function out = timestamp()
%TIMESTAMP Showing timestamps to make sure it is not a tic/toc problem
out = datestr(now, 'HH:MM:SS.FFF');

%%
function out = systemDescription()
if ispc
    platform = [system_dependent('getos'),' ',system_dependent('getwinsys')];
elseif ismac
    [fail, input] = unix('sw_vers');
    if ~fail
        platform = strrep(input, 'ProductName:', '');
        platform = strrep(platform, sprintf('\t'), '');
        platform = strrep(platform, sprintf('\n'), ' ');
        platform = strrep(platform, 'ProductVersion:', ' Version: ');
        platform = strrep(platform, 'BuildVersion:', 'Build: ');
    else
        platform = system_dependent('getos');
    end
else
    platform = system_dependent('getos');
end
arch = getenv('PROCESSOR_ARCHITEW6432');
if isempty(arch)
    arch = getenv('PROCESSOR_ARCHITECTURE');
end
try
    [~,sysMem] = memory();
catch
    sysMem.PhysicalMemory.Total = NaN;
end
out = sprintf('%s, arch=%s, %.0f GB, os=%s',...
    computer, arch, sysMem.PhysicalMemory.Total/(2^30), platform);

%%
function out = setupTempDir()
out = fullfile(tempdir, sprintf('%s - %s', mfilename, datestr(now, 'yyyymmdd-HHMMSS-FFF')));
mkdir(out);

编辑：我修改了repro函数，添加了多次迭代并将其参数化为randi生成器的保存样式，文件格式和imax。

我认为文件系统缓存是快速重叠行为的一个重要因素。当我使用reproMatfileAppendSpeedup（20）连续运行一堆并在Process Explorer中查看系统信息时，大多数都在一秒之内，并且物理内存使用量会快速增加几GB。然后每十几次传递，写入停顿并花费20或30秒，并且物理RAM使用率缓慢下降到大约开始的位置。我认为这意味着Windows在RAM中缓存了大量写入，并且有关-append的内容使其更愿意这样做。但对于我来说，包括那些摊位在内的摊还时间仍比基本保存快得多。

顺便说一下，经过几个小时的多次通过后，我很难再现原始时间。