[blog] Advanced memory management
Being an emulator of a fairly robust system (the PS2), PCSX2 typically consumes a lot of system RAM. It needs multitudes of caches and buffers for various things. Just to give an idea, I'll list some of the larger stuff and their current defaults:
  • PS2 main memory [32mb]
  • IOP memory [2mb]
  • EE/IOP BIOS roms [6mb]
  • Scratchpad, Hardware registers, VU memory, DMA buffers, etc [4mb]
  • VTLB indexes, lookups, and protection tables [8mb]
  • EE/R5900 recompiler cache [16mb]
  • EE/R5900 recompiler block/pc translation table [48mb]
  • R5900 memory protection mirror [32mb]
  • IOP/R3000A recompiler cache and translation table [10mb]
  • microVU recompiler code caches [16mb * 2]
  • superVU recompiler code caches [8mb]

If all of these things are reserved when PCSX2 starts, we have a base memory footprint of over 200 megs before even a single instruction of PS2 code is executed! The worst part is that we could really stand to allocate even more ram: some games need over 120 mb of recompiler caches to run properly. Currently those games are dealt with by issuing periodic recompiler resets (sluggish).

Fortunately modern operating systems have a lot of built-in features that help us out. Both Windows and Linux OSes use virtual memory mapping features of our Intel/AMD cpus to perform "virtual" allocations of large memory reserves. What this means is that initially the allocated memory has no actual physical equivalent. It is only given a physical presence once the memory is accessed (read or written). Explained as a process:

1. App requests 1gb of RAM via malloc.
2. Operating system "reserves" the 1gb of RAM, which marks the virtual addresses for use by this memory only. In this case the memory might be reserved from 0x10000000 (0.4gb) -> 0x50000000 (1.4gb).
3. Operating system "commits" the 1gb of RAM, ensuring there is enough physical and swapfile RAM to accommodate it. No actual memory or swapfile changes are made; only the tracked amount of ram/swap in reserve is altered.
3. App receives a pointer to the reserved ram.
4. App reads or writes data -- 128mb worth, let's say.
5. OS receives a page fault exception for that memory, and allocates a chunk of physical RAM for it. Other processes may be swapped out to disk at this time to make room for the memory in-use.

Only at Point 5 does any actual physical ram get used by the program. Prior to Point 5, the app has used exactly 0 byte of RAM, in spite of allocating 1gb via malloc. This feature is implicit to both Windows and Linux and already helps work wonders on PCSX2's overall memory footprint. This is also why you might get "Low on virtual memory!" errors even though it appears as though you have lots of free ram in the System Monitor / Process Explorer, because some apps commit lots of memory but only actually access a small fraction of it.

There are ways, however, to fine tune memory access and get even better memory management than the implicit Windows / Linux provisions via malloc. The first rule is a simple one, but one many programmers probably have no idea about it:

Do not use calloc, and do not clear allocated memory by default unless you absolutely have to!

Calling calloc instead of malloc causes the entire allocation (1gb in our above example) to be committed to physical memory because of its being cleared to zero. Likewise, manually clearing buffers to zero (or some other value) has the same effect. Even if only a small portion of the array ends up being used later on, its too late: the whole thing is sucking up resources for no good reason except to express a patterned fill value. Sometimes clearing buffers cannot be avoided, but most of the time buffers need not be cleared at all, and programmers simply use calloc or manual clears out of habit.

Using Reserve and Commit to manage recompiled code buffers.

There are two phases to allocating memory on a virtual memory system, as noted in the small ordered list above. By default, malloc will reserve and commit ram together. This is done so that the system can ensure that there is enough ram and swap to free to give the program the entire allocation -- if it happens to ever need it. If the commit phase fails due to there not being enough physical ram, malloc returns NULL. If you manage the reserve and commit phases separately, then you can reserve extra large swatches of memory addresses without affecting the rest of the operating system in any way; and then later on commit portions of the reserve only as needed. There aren't a whole lot of reasons why you'd need to micro-manage the virtual memory system in this way, and for most purposes simply using malloc and letting the OS do its own internal management suffices nicely. Lucky for us, PCSX2 has one!

One of the troubles with recompiled code is that it can't be allowed to move. Typically use of malloc and realloc results in allocated memory moving around as it grows or shrinks. This is fine for most purposes, but is disastrous to executable code since it invalidates all block pointers and long jumps (which use absolute addressing). In order to grow a recompiled code cache using traditional malloc, you have to clear the cache and start over -- a recompiler reset. This usually causes a lengthy hiccup in emulation speed when it happens.

Virtual memory techniques can be used to get around that. When we reserve the recompiled code cache, we reserve the upper limit of what we deem a sane cache size. In this case, the R5900 cache should be a maximum of 48mb. The 48mb is reserved from 0x30000000->0x33000000 when PCSX2 starts, the first 4mb are committed when PCSX2 starts executing R5900 code. When the cache fills, PCSX2 automatically commits more memory in 128k increments, up to 48mb -- at which point the emulator will reset the cache and start over. Thanks to the virtual memory strategy described above, only a fraction of the 48meg allocation actually exists in physical ram unless more of the allocation is actually needed. Furthermore, computers with limited RAM resources or disabled swapfiles will still be able to run PCSX2 nicely.

Committing blocks of memory from the 48meg reserve never alters the base address of the memory, so no pointers become invalid, and no memory needs to be copied or shuffled in order to make room for the larger caches. The end result is near instantaneous increases in cache size, on-the-fly! ... and all-the-while maintaining compact and efficient memory footprint for games that don't need more than the basic caches.

On Windows this technique is implemented using VirtualAlloc, which is fairly well documented via the linked MSDN page. On Linux, however, things get a bit strange. The technique can be implemented using a combination of mmap and mprotect, but unfortunately the Linux man pages lack any actual explanation of how to perform independent reserve and commit actions (but rest assured, it can be done). Furthermore, Linux has an implicit system enabled by default called Over-committing, which basically skips phase (3) described above -- and always returns a valid pointer on calls to malloc, even if the system hasn't enough ram to accomodate the request.

Over-committing is so surpassingly hacky and evil that it deserves a blog post all to itself, so stay tuned. Wink

To be continued...
Jake Stine (Air) - Programmer - PCSX2 Dev Team

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