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Re: [Qemu-devel] [PATCH v5 00/18] tb hash improvements

From: Sergey Fedorov
Subject: Re: [Qemu-devel] [PATCH v5 00/18] tb hash improvements
Date: Tue, 24 May 2016 01:26:54 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

I think I'm done reviewing v5. (Though I haven't reviewed tests and
statistics patches.)

Kind regards,

On 14/05/16 06:34, Emilio G. Cota wrote:
> This patchset applies on top of tcg-next (8b1fe3f4 "cpu-exec:
> Clean up 'interrupt_request' reloading", tagged "pull-tcg-20160512").
> For reference, here is v4:
>   https://lists.gnu.org/archive/html/qemu-devel/2016-04/msg04670.html
> Changes from v4:
> - atomics.h:
>   + Add atomic_read_acquire and atomic_set_release
>   + Rename atomic_test_and_set to atomic_test_and_set_acquire
>   [ Richard: I removed your reviewed-by ]
> - qemu_spin @ thread.h:
>   + add bool qemu_spin_locked() to check whether the lock is taken.
>   + Use newly-added acquire/release atomic ops. This is clearer and
>     improves performance; for instance, now we don't emit an
>     unnecessary smp_mb() thanks to using atomic_set_release()
>     instead of atomic_mb_set(). Also, note that __sync_test_and_set
>     has acquire semantics, so it makes sense to have an
>     atomic_test_and_set_acquire that directly calls it, instead
>     of calling atomic_xchg, which emits a full barrier (that we don't
>     need) before __sync_test_and_set.
>   [ Richard: I removed your reviewed-by ]
> - tests:
>   + add parallel benchmark (qht-bench). Some perf numbers in
>     the commit message, comparing QHT vs. CLHT and ck_hs.
>   + invoke qht-bench from `make check` with test-qht-par. It
>     uses system(3); I couldn't find a way to detect from qht-bench
>     when it is run from gtester, so I decided to just add a silly
>     program to invoke it.
> - trivial: util/Makefile.objs: add qdist.o and qht.o each on a
>            separate line
> - trivial: added copyright header to test programs
> - trivial: updated phys_pc, pc, flags commit message with Richard's
>            comment that hashing cs_base probably isn't worth it.
> - qht:
>   + Document that duplicate pointer values cannot be inserted.
>   + qht_insert: return true/false upon success/failure, just like
>                 qht_remove. This can help find bugs.
>   + qht_remove: only write to seqlock if the removal happens --
>                 otherwise the write is unnecessary, since nothing
>               is written to the bucket.
>   + trivial: s/n_items/n_entries/ for consistency.
>   + qht_grow: substitute it for qht_resize. This is mostly useful
>               for testing.
>   + resize: do not track qht_map->n_entries; track instead the
>             number of non-head buckets added.
>           This improves scalability, since we only increment
>           this number (with the relatively expensive atomic_inc)
>           every time a new non-head bucket is allocated, instead
>           of every time an entry is added/removed.
>     * return bool from qht_resize and qht_reset_size; they return
>       false if the resize was not needed (i.e. if the previous size
>       was the requested size).
>   + qht_lookup: do not check for !NULL entries; check directly
>                 for a hash match.
>               This gives a ~2% perf. increase during
>               benchmarking. The buckets in the microbenchmarks
>               are equally-sized well distributed, which is
>               approximately the case in QEMU thanks to xxhash
>               and resizing.
>   + Remove MRU bucket promotion policy. With automatic resizing,
>     this is not needed. Furthermore, removing it saves code.
>   + qht_lookup: Add fast-path without do {} while (seqlock). This
>                 gives a 4% perf. improvement on a read-only benchmark.
>   + struct qht_bucket: document the struct
>   + rename qht_lock() to qht_map_lock_buckets()
>   + add map__atomic_mb and bucket_next__atomic_mb helpers that
>     include the necessary atomic_read() and rmb().
>   [ All the above changes for qht are simple enough that I kept
>     Richard's reviewed-by.]
>   + Support concurrent writes to separate buckets. This is in an
>     additional patch to ease reviewing; feel free to squash it on
>     top of the QHT patch.
> Thanks,
>               Emilio

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