linux-kernel - Re: [patch 02/20] make the inode i_mmap

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Message-Id: <200712191148.06506.nickpiggin@yahoo.com.au>
Date:	Wed, 19 Dec 2007 11:48:06 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	lee.shermerhorn@...com, Lee Schermerhorn <lee.schermerhorn@...com>
Subject: Re: [patch 02/20] make the inode i_mmap_lock a reader/writer lock

On Wednesday 19 December 2007 08:15, Rik van Riel wrote:
> I have seen soft cpu lockups in page_referenced_file() due to
> contention on i_mmap_lock() for different pages.  Making the
> i_mmap_lock a reader/writer lock should increase parallelism
> in vmscan for file back pages mapped into many address spaces.
>
> Read lock the i_mmap_lock for all usage except:
>
> 1) mmap/munmap:  linking vma into i_mmap prio_tree or removing
> 2) unmap_mapping_range:   protecting vm_truncate_count
>
> rmap:  try_to_unmap_file() required new cond_resched_rwlock().
> To reduce code duplication, I recast cond_resched_lock() as a
> [static inline] wrapper around reworked cond_sched_lock() =>
> __cond_resched_lock(void *lock, int type).
> New cond_resched_rwlock() implemented as another wrapper.

Reader/writer locks really suck in terms of fairness and starvation,
especially when the read-side is common and frequent. (also, single
threaded performance of the read-side is worse).

I know Lee saw some big latencies on the anon_vma list lock when
running (IIRC) a large benchmark... but are there more realistic
situations where this is a problem?
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