How to un-eat memory?

Consider this code:

MemoryInUse[]
T = Table[RandomComplex[], {i, 1, 6000}, {j, 1, 6000}];
MemoryInUse[]
T += T\[ConjugateTranspose];
MemoryInUse[]
{Es, Ys} = Eigensystem[T];
MemoryInUse[]

T = Table[RandomComplex[], {i, 1, 6000}, {j, 1, 6000}];
MemoryInUse[]
T += T\[ConjugateTranspose];
MemoryInUse[]
{Es, Ys} = Eigensystem[T];
MemoryInUse[]
$HistoryLength = 0;
MemoryInUse[]
Clear[T]
MemoryInUse[]

Clear[Es, Ys]
MemoryInUse[]
ClearSystemCache[]
MemoryInUse[]

It gives me the following results:

15808208

880820520

1456822832

4919500424

5783503032

6359505096

9822181440

9822182648

9822182112

9822181384

9822162952

Clearly, the memory clears negligibly on any of ClearSystemCache, Clear and zeroing $HistoryLength. Repeating its execution leads to swapping, after start of which I hurry up to kill MathKernel before my X or WM or anything else are OOM-killed.

So what are the working ways to release the memory?

Answer

$HistoryLength is just a global variable. It not clear the history. You can still access the history by

ByteCount@Out[12]
ByteCount@%10

It can be cleared by

Unprotect[In, Out]
Clear[In, Out]

However, it would be better if you set $HistoryLength=0 before your resource-intensive code.

P.S. It would be great to have $HistoryMemoryLimit or something like this.