Too little, too slow

An introduction to memory management
Linux memory management today and tomorrow

Rik van Riel
Conectiva S.A.

Generated from the Magicpoint slides 27/nov/2000
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Too little, too slow

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Memory hierarchy


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Memory hierarchy: basics

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Memory hierarchy: who does what?

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Memory hierarchy: numbers

Below is a table with some "randomly" chosen CPU types and their L1 and L2 cache characteristics. The exact numbers are not very important, but it is good to realise that even the L2 cache is not fast enough to let the CPU core run at 4% of its maximum speed...

Cache size & speed vs. CPU model Celeron A PIII Athlon K6-III
L1 size 32kB 32kB 128kB 64kB
L2 size 128kB 512kB 512kB 256kB
L1 lat 3 3 3 2
L2 lat 8 24 21 11
L2 total 11 27 24 13
L1 ass. 4 4 2 2
L2 ass. 8 4 2 4

As you can see, RAM and hard disk speeds are so rediculously low that you must avoid RAM and disk access at all cost if you want performance.

RAM latency: 50 - 400 cycles
Disk latency: > 3.000.000 cycles

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Memory hierarchy: working set

Of course this breaks in applications that don't exhibit good locality \
of reference. Luckily it works well for most programs.

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Cache: how it works

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Cache: dirty memory

The fastest way of getting a cache line replaced is by not writing \
to it, so we CAN just forget about it when we need to cache something \

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Cache: coherency

We must make sure that we won't write stale data to disk or work on \
stale data from another CPU.

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Cache: program optimisation

This example is from the Linux kernel source code, from \

When all running processes have used their time slice:

		p->counter = (p->counter >> 1) + p->priority;

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Cache: program optimisation

The magic trick:

		p->counter = (p->counter >> 1) + p->priority;

	for_each_task(p) {
		if (p->counter != (p->counter >> 1) + p->priority)
			p->counter = (p->counter >> 1) + p->priority;

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RAM: virtual memory

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RAM: virtual memory


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RAM: virt -> phys mapping

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RAM: page faults

Because a page fault is very Very VERY slow, we really want to \
minimise the amount of page faults.

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RAM: page replacement

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RAM: LRU approximations

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RAM: other replacement tricks

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Disk: performance characteristics

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Disk: performance optimisations

How to get disk performance good?

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Disk: RAID

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HSM: beyond disk

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Too little, too slow

Conclusions of the introduction

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Linux Memory Management

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Linux VM: physical zones

actions like swapin and swapout

On x86 the zones happen to be "inclusive", but this is not the case for \
some other architectures (eg. with NUMA-like setups).

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Linux VM: virtual page use

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Linux VM 2.2: try_to_free_pages

You can read the functions in mm/vmscan.c and mm/filemap.c

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Linux 2.2 VM: strong / weak points

we haven't scanned memory in a long time)

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Linux VM: changes in 2.4

Not yet in 2.4 as of september 2000

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Linux VM: 2.4 / 2.5 page aging

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Linux VM: multiqueue VM

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Linux VM: balancing aging and flushing

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Linux 2.4 VM: try_to_free_pages

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Linux 2.4 VM: page_launder

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Linux VM: flush callback & pinned pages

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Linux VM: plans for 2.5

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Linux VM: plans for 2.5

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Linux VM: more plans for 2.5

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Linux VM: OOM / QoS

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Thanks go out to

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