Re: [PATCH v2 01/13] mm: support madvise(MADV_FREE)

[Daniel Micay]

> Compared to MADV_DONTNEED, MADV_FREE's lazy memory free is a huge win to reduce
> page fault. But there is one issue remaining, the TLB flush. Both MADV_DONTNEED
> and MADV_FREE do TLB flush. TLB flush overhead is quite big in contemporary
> multi-thread applications. In our production workload, we observed 80% CPU
> spending on TLB flush triggered by jemalloc madvise(MADV_DONTNEED) sometimes.
> We haven't tested MADV_FREE yet, but the result should be similar. It's hard to
> avoid the TLB flush issue with MADV_FREE, because it helps avoid data
> corruption.
> 
> The new proposal tries to fix the TLB issue. We introduce two madvise verbs:
> 
> MARK_FREE. Userspace notifies kernel the memory range can be discarded. Kernel
> just records the range in current stage. Should memory pressure happen, page
> reclaim can free the memory directly regardless the pte state.
> 
> MARK_NOFREE. Userspace notifies kernel the memory range will be reused soon.
> Kernel deletes the record and prevents page reclaim discards the memory. If the
> memory isn't reclaimed, userspace will access the old memory, otherwise do
> normal page fault handling.
> 
> The point is to let userspace notify kernel if memory can be discarded, instead
> of depending on pte dirty bit used by MADV_FREE. With these, no TLB flush is
> required till page reclaim actually frees the memory (page reclaim need do the
> TLB flush for MADV_FREE too). It still preserves the lazy memory free merit of
> MADV_FREE.
> 
> Compared to MADV_FREE, reusing memory with the new proposal isn't transparent,
> eg must call MARK_NOFREE. But it's easy to utilize the new API in jemalloc.
> 
> We don't have code to backup this yet, sorry. We'd like to discuss it if it
> makes sense.

That's comparable to Android's pinning / unpinning API for ashmem and I
think it makes sense if it's faster. It's different than the MADV_FREE
API though, because the new allocations that are handed out won't have
the usual lazy commit which MADV_FREE provides. Pages in an allocation
that's handed out can still be dropped until they are actually written
to. It's considered active by jemalloc either way, but only a subset of
the active pages are actually committed. There's probably a use case for
both of these systems.

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