时间仓促没有时间排版
公司内部大型case回归中出现了内存增加的问题，最初怀疑是代码变动导致，根据tcmalloc内存池的实时占用打印最终分析出来是tcl命令string first单次操作增加了GB级别的内存，一起来看一下是怎么个事

测试条件准备

一个1G的字符串和一个2G的字符串

两个标准的tclsh，tcl8.6与tcl9.0.
测试脚本，此脚本读入case，从/proc/self/status获取真实内存消耗，分别计算处理后的内存占用。

# mem_test_default.tcl
proc rss {} {
    set f [open /proc/self/status r]
    set data [read $f]
    close $f
    foreach line [split $data "\n"] {
        if {[string match "VmRSS:*" $line]} {
            return [string trim $line]
        }
    }
}

set path [lindex $argv 0]

puts "==== start ===="
puts [rss]
puts "tcl_platform(encoding): $tcl_platform(byteOrder) / default system encoding: [encoding system]"

# 完全默认方式:不指定 -encoding,不用 rb,不手动 convert
set fh [open $path]
set s [read $fh]
close $fh

puts "==== after default read ===="
puts [rss]
puts "string length (chars): [string length $s]"
puts "repr: [tcl::unsupported::representation $s]"

set idx [string first "THIS_PATTERN_PROBABLY_NOT_IN_FILE_XYZ" $s]
puts "==== after string first (idx=$idx) ===="
puts [rss]
puts "repr: [tcl::unsupported::representation $s]"

使用方法

tclsh8.6 mem_test.tcl /path/to/1g_file.txt
tclsh9.0 mem_test.tcl /path/to/1g_file.txt

tclsh8.6 mem_test.tcl /path/to/2g_file.txt
tclsh9.0 mem_test.tcl /path/to/2g_file.txt

测试结果

tcl8.6-2g

==== start ====
VmRSS:      2560 kB
tcl_platform(encoding): littleEndian / default system encoding: utf-8
==== after default read ====
VmRSS:   1912320 kB
string length (chars): 1957780176
repr: value is a string with a refcount of 2, object pointer at 0x1599f5c0, internal representation 0x159b45b0:0x1599f260, string representation "This software..."
==== after string first (idx=-1) ====
VmRSS:   5734400 kB
repr: value is a string with a refcount of 2, object pointer at 0x1599f5c0, internal representation 0x7f47bcc0a020:0x1599f260, string representation "This software..."

tcl9.0-2g

==== start ====
VmRSS:      3840 kB
tcl_platform(encoding): littleEndian / default system encoding: utf-8
==== after default read ====
VmRSS:   1916160 kB
string length (chars): 1957780176
repr: value is a string with a refcount of 2, object pointer at 0x6ac8310, internal representation 0x6af3200:0x0, string representation "This software..."
==== after string first (idx=-1) ====
VmRSS:   9561600 kB
repr: value is a string with a refcount of 2, object pointer at 0x6ac8310, internal representation 0x7fc96b1ba020:0x0, string representation "This software..."

可以看到明显内存的增加，增加的量
9561600 -5734400 =3,827,200

好巧不巧
3,827,200 / 1000000 /2 =1.9136 G

等于文件的大小