Re: Can context switches be faster?

[John Richard Moser]

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Andrew James Wade wrote:
> On Thursday 12 October 2006 14:29, John Richard Moser wrote:
>> How does a page table switch work?  As I understand there are PTE chains
>> which are pretty much linked lists the MMU follows; I can't imagine this
>> being a harder problem than replacing the head.
> 
> Generally, the virtual memory mappings are stored as high-fanout trees
> rather than linked lists. (ia64 supports a hash table based scheme,
> but I don't know if Linux uses it.)  But the bulk of the mapping
> lookups will actually occur in a cache of the virtual memory mappings
> called the translation lookaside buffer (TLB). It is from the TLB and
> not the memory mapping trees that some of the performance problems
> with address space switches originate.
> 
> The kernel can tolerate some small inconsistencies between the TLB
> and the mapping tree (it can fix them in the page fault handler). But
> for the most part the TLB must be kept consistent with the current
> address space mappings for correct operation. Unfortunately, on some
> architectures the only practical way of doing this is to flush the TLB
> on address space switches. I do not know if the flush itself takes any
> appreciable time, but each of the subsequent TLB cache misses will
> necessitate walking the current mapping tree. Whether done by the MMU
> or by the kernel (implementations vary), these walks in the aggregate
> can be a performance issue.

True.  You can trick the MMU into faulting into the kernel (PaX does
this to apply non-executable pages-- pages, not halves of VM-- on x86),
but it's orders of magnitude slower as I understand and the petty gains
you can get over the hardware MMU doing it are not going to outweigh it.

> 
> On some architectures the L1 cache can also require attention from the
> kernel on address space switches for correct operation. Even when the
> L1 cache doesn't need flushing a change in address space will generally
> be accompanied by a change of working set, leading to a period of high
> cache misses for the L1/L2 caches. 

Yeah, only exception being if L1 and L2 are both physically addressed,
and thing like libc's .text are shared, leading to shared working sets
in I1 and L2.

> 
> Microbenchmarks can miss the cache miss costs associated with context
> switches. But I believe the costs of cache thrashing and flushing are

cachegrind is probably guilty but I haven't examined it.

> the reason that the time-sharing granularity is so coarse in Linux,
> rather than the time it takes the kernel to actually perform a context
> switch. (The default time-slice is 100 ms.) Still, the cache miss costs

I thought it was minimum 5mS... I don't know what default is.  Heh.

> are workload-dependent, and the actual time the kernel takes to context
> switch can be important as well.
> 
> Andrew Wade
> 

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