[FFmpeg-devel] [PATCH 1/3] avutil: add staticpool

[Rostislav Pehlivanov]

On 20 January 2018 at 10:22, wm4 <nfxjfg at googlemail.com> wrote:

> On Sat, 20 Jan 2018 11:29:13 +0700
> Muhammad Faiz <mfcc64 at gmail.com> wrote:
>
> > Help avoiding malloc-free cycles when allocating-freeing common
> > structures.
> >
> > Signed-off-by: Muhammad Faiz <mfcc64 at gmail.com>
> > ---
> >  libavutil/staticpool.h | 117 ++++++++++++++++++++++++++++++
> +++++++++++++++++++
> >  1 file changed, 117 insertions(+)
> >  create mode 100644 libavutil/staticpool.h
> >
> > diff --git a/libavutil/staticpool.h b/libavutil/staticpool.h
> > new file mode 100644
> > index 0000000000..9c9b2784bc
> > --- /dev/null
> > +++ b/libavutil/staticpool.h
> > @@ -0,0 +1,117 @@
> > +/*
> > + * This file is part of FFmpeg.
> > + *
> > + * FFmpeg is free software; you can redistribute it and/or
> > + * modify it under the terms of the GNU Lesser General Public
> > + * License as published by the Free Software Foundation; either
> > + * version 2.1 of the License, or (at your option) any later version.
> > + *
> > + * FFmpeg is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> > + * Lesser General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU Lesser General Public
> > + * License along with FFmpeg; if not, write to the Free Software
> > + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
> 02110-1301 USA
> > + */
> > +
> > +#ifndef AVUTIL_STATICPOOL_H
> > +#define AVUTIL_STATICPOOL_H
> > +
> > +#include <stdatomic.h>
> > +#include "avassert.h"
> > +#include "mem.h"
> > +
> > +/**
> > + * FF_STATICPOOL allocate memory without av_malloc if possible
> > + * @param size must be 2^n between 64 and 4096
> > + */
> > +#define FF_STATICPOOL_DECLARE(type, size)
>          \
> > +typedef struct type##_StaticPoolWrapper {
>          \
> > +    type        buf;
>         \
> > +    unsigned    index;
>         \
> > +    atomic_uint next;
>          \
> > +} type##_StaticPoolWrapper;
>          \
> > +
>         \
> > +static atomic_uint              type##_staticpool_next;
>          \
> > +static atomic_uint              type##_staticpool_last;
>          \
> > +static type##_StaticPoolWrapper type##_staticpool_table[size];
>         \
> > +
>         \
> > +static type *type##_staticpool_malloc(void)
>          \
> > +{
>          \
> > +    unsigned val, index, serial, new_val;
>          \
> > +
>         \
> > +    av_assert0((size) >= 64 && (size) <= 4096 && !((size) & ((size) -
> 1)));     \
> > +
>         \
> > +    /* use serial, avoid spinlock */
>         \
> > +    /* acquire, so we don't get stalled table[index].next */
>         \
> > +    val = atomic_load_explicit(&type##_staticpool_next,
> memory_order_acquire);  \
> > +    do {
>         \
> > +        index  = val & ((size) - 1);
>         \
> > +        serial = val & ~((size) - 1);
>          \
> > +        new_val = atomic_load_explicit(&type##_staticpool_table[index].next,
> memory_order_relaxed) | (serial + (size)); \
> > +    } while (!atomic_compare_exchange_strong_explicit(&type##_staticpool_next,
> &val, new_val, \
> > +
> memory_order_acquire, memory_order_acquire)); \
> > +
>         \
> > +    index = val & ((size) - 1);
>          \
> > +    if (index)
>         \
> > +        return &type##_staticpool_table[index].buf;
>          \
> > +
>         \
> > +    index = atomic_fetch_add_explicit(&type##_staticpool_last, 1,
> memory_order_relaxed) + 1; \
> > +    if (index < (size)) {
>          \
> > +        type##_staticpool_table[index].index = index;
>          \
> > +        return &type##_staticpool_table[index].buf;
>          \
> > +    }
>          \
> > +
>         \
> > +    atomic_fetch_add_explicit(&type##_staticpool_last, -1,
> memory_order_relaxed); \
> > +    return av_malloc(sizeof(type));
>          \
> > +}
>          \
> > +
>         \
> > +static inline type *type##_staticpool_mallocz(void)
>          \
> > +{
>          \
> > +    type *ptr = type##_staticpool_malloc();
>          \
> > +    if (ptr)
>         \
> > +        memset(ptr, 0, sizeof(*ptr));
>          \
> > +    return ptr;
>          \
> > +}
>          \
> > +
>         \
> > +static void type##_staticpool_free(type *ptr)
>          \
> > +{
>          \
> > +    type##_StaticPoolWrapper *entry = (type##_StaticPoolWrapper *)
> ptr;         \
> > +    unsigned val, serial, index, new_val;
>          \
> > +
>         \
> > +    if ((uintptr_t)ptr <= (uintptr_t)(type##_staticpool_table) ||
>          \
> > +        (uintptr_t)ptr >= (uintptr_t)(type##_staticpool_table + size))
> {        \
> > +        av_free(ptr);
>          \
> > +        return;
>          \
> > +    }
>          \
> > +
>         \
> > +    if (CONFIG_MEMORY_POISONING)
>         \
> > +        memset(&entry->buf, FF_MEMORY_POISON, sizeof(entry->buf));
>         \
> > +
>         \
> > +    val = atomic_load_explicit(&type##_staticpool_next,
> memory_order_relaxed);  \
> > +    do {
>         \
> > +        index  = val & ((size) - 1);
>         \
> > +        serial = val & ~((size) - 1);
>          \
> > +        atomic_store_explicit(&entry->next, index,
> memory_order_relaxed);       \
> > +        new_val = entry->index | (serial + (size));
>          \
> > +    } while (!atomic_compare_exchange_strong_explicit(&type##_staticpool_next,
> &val, new_val, \
> > +
> memory_order_release, memory_order_relaxed)); \
> > +}
>          \
> > +
>         \
> > +static inline void type##_staticpool_freep(type **ptr)
>         \
> > +{
>          \
> > +    if (ptr) {
>         \
> > +        type##_staticpool_free(*ptr);
>          \
> > +        *ptr = NULL;
>         \
> > +    }
>          \
> > +}
>          \
> > +/* FF_STATICPOOL_DECLARE */
> > +
> > +#define FF_STATICPOOL_MALLOC(type) type##_staticpool_malloc()
> > +#define FF_STATICPOOL_MALLOCZ(type) type##_staticpool_mallocz()
> > +#define FF_STATICPOOL_FREE(type, v) type##_staticpool_free(v)
> > +#define FF_STATICPOOL_FREEP(type, v) type##_staticpool_freep(v)
> > +
> > +#endif
>
> I'm against this. Global mutable data is always bad. If you have 2
> uses ffmpeg in the same process, their efficiency might get halved
> because the available pool size is shared by both. The macro mess is
> very ugly. Can we please not define pages of code as macros. Last but
> not least this is the job of the memory manager, which already does
> fancy things like per thread pools. I don't trust the atomics use
> either, I'm don't want to have to debug that ever.
>
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> ffmpeg-devel at ffmpeg.org
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>


I agree.
Also the last GCC version has multithreaded malloc cache already
implemented so it makes this patch only useful on non-glibc systems (or
systems which don't implement malloc caching).