[v2] mbuf: use C11 atomics for refcnt operations
Checks
Commit Message
Use C11 atomics with explicit ordering instead of rte_atomic ops which
enforce unnecessary barriers on aarch64.
Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
---
v2:
Fix ABI issue: revert the rte_mbuf_ext_shared_info struct refcnt field
to refcnt_atomic.
lib/librte_mbuf/rte_mbuf.c | 1 -
lib/librte_mbuf/rte_mbuf.h | 19 ++++++++++---------
lib/librte_mbuf/rte_mbuf_core.h | 11 +++--------
3 files changed, 13 insertions(+), 18 deletions(-)
Comments
On Tue, 7 Jul 2020 18:10:33 +0800
Phil Yang <phil.yang@arm.com> wrote:
> + return (uint16_t)(__atomic_add_fetch((int16_t *)&shinfo->refcnt_atomic,
> +
Why do you need so many casts here?
The type of refcnt_atomic is now uint16 after your patch.
> -----Original Message-----
> From: Stephen Hemminger <stephen@networkplumber.org>
> Sent: Wednesday, July 8, 2020 1:13 AM
> To: Phil Yang <Phil.Yang@arm.com>
> Cc: david.marchand@redhat.com; dev@dpdk.org; drc@linux.vnet.ibm.com;
> Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> olivier.matz@6wind.com; Ruifeng Wang <Ruifeng.Wang@arm.com>; nd
> <nd@arm.com>
> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt
> operations
>
> On Tue, 7 Jul 2020 18:10:33 +0800
> Phil Yang <phil.yang@arm.com> wrote:
>
> > + return (uint16_t)(__atomic_add_fetch((int16_t *)&shinfo-
> >refcnt_atomic,
> > +
>
> Why do you need so many casts here?
> The type of refcnt_atomic is now uint16 after your patch.
In the existing code, the input parameter type for this API is signed integer. For example:
drivers/net/netvsc/hn_rxtx.c:531
lib/librte_mbuf/rte_mbuf.h:1194
However, the output type of rte_mbuf_ext_refcnt related APIs is not uniform. We use these typecast to consistent with the current API definition.
> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Phil Yang
> Sent: Tuesday, July 7, 2020 6:11 PM
> To: david.marchand@redhat.com; dev@dpdk.org
> Cc: drc@linux.vnet.ibm.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; olivier.matz@6wind.com; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> Subject: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt operations
>
> Use C11 atomics with explicit ordering instead of rte_atomic ops which
> enforce unnecessary barriers on aarch64.
>
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
> v2:
> Fix ABI issue: revert the rte_mbuf_ext_shared_info struct refcnt field
> to refcnt_atomic.
>
<snip>
> diff --git a/lib/librte_mbuf/rte_mbuf_core.h
> b/lib/librte_mbuf/rte_mbuf_core.h
> index 16600f1..806313a 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -18,7 +18,6 @@
>
> #include <stdint.h>
> #include <rte_compat.h>
> -#include <generic/rte_atomic.h>
>
> #ifdef __cplusplus
> extern "C" {
> @@ -495,12 +494,8 @@ struct rte_mbuf {
> * or non-atomic) is controlled by the
> CONFIG_RTE_MBUF_REFCNT_ATOMIC
> * config option.
> */
> - RTE_STD_C11
> - union {
> - rte_atomic16_t refcnt_atomic; /**< Atomically accessed
> refcnt */
> - /** Non-atomically accessed refcnt */
> - uint16_t refcnt;
> - };
> + uint16_t refcnt;
> +
> uint16_t nb_segs; /**< Number of segments. */
>
> /** Input port (16 bits to support more than 256 virtual ports).
> @@ -679,7 +674,7 @@ typedef void
> (*rte_mbuf_extbuf_free_callback_t)(void *addr, void *opaque);
> struct rte_mbuf_ext_shared_info {
> rte_mbuf_extbuf_free_callback_t free_cb; /**< Free callback
> function */
> void *fcb_opaque; /**< Free callback argument */
> - rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
> + uint16_t refcnt_atomic; /**< Atomically accessed refcnt */
It still causes an ABI check failure in Travis CI on this type change.
I think we need an exception in libabigail.abignore for this.
Thanks,
Phil
> };
>
> /**< Maximum number of nb_segs allowed. */
> --
> 2.7.4
On Wed, Jul 08, 2020 at 04:48:33AM +0000, Phil Yang wrote:
> > -----Original Message-----
> > From: Stephen Hemminger <stephen@networkplumber.org>
> > Sent: Wednesday, July 8, 2020 1:13 AM
> > To: Phil Yang <Phil.Yang@arm.com>
> > Cc: david.marchand@redhat.com; dev@dpdk.org; drc@linux.vnet.ibm.com;
> > Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> > olivier.matz@6wind.com; Ruifeng Wang <Ruifeng.Wang@arm.com>; nd
> > <nd@arm.com>
> > Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt
> > operations
> >
> > On Tue, 7 Jul 2020 18:10:33 +0800
> > Phil Yang <phil.yang@arm.com> wrote:
> >
> > > + return (uint16_t)(__atomic_add_fetch((int16_t *)&shinfo-
> > >refcnt_atomic,
> > > +
> >
> > Why do you need so many casts here?
> > The type of refcnt_atomic is now uint16 after your patch.
>
> In the existing code, the input parameter type for this API is signed integer. For example:
> drivers/net/netvsc/hn_rxtx.c:531
> lib/librte_mbuf/rte_mbuf.h:1194
>
> However, the output type of rte_mbuf_ext_refcnt related APIs is not uniform. We use these typecast to consistent with the current API definition.
Would it make sense to cast the increment instead?
I mean:
return __atomic_add_fetch(&m->refcnt, (uint16_t)value, __ATOMIC_ACQ_REL);
instead of:
return (uint16_t)(__atomic_add_fetch((int16_t *)&m->refcnt, value,
__ATOMIC_ACQ_REL));
The same could apply to __rte_pktmbuf_pinned_extbuf_decref() I think:
> - if (likely(rte_atomic16_add_return
> - (&shinfo->refcnt_atomic, -1)))
> + if (likely(__atomic_add_fetch((int *)&shinfo->refcnt_atomic, -1,
> + __ATOMIC_ACQ_REL)))
By the way, why was the cast was to (int *) in this case? I think
it can overwrite fields beside refcnt.
Hi,
On Tue, Jul 07, 2020 at 06:10:33PM +0800, Phil Yang wrote:
> Use C11 atomics with explicit ordering instead of rte_atomic ops which
> enforce unnecessary barriers on aarch64.
>
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
> v2:
> Fix ABI issue: revert the rte_mbuf_ext_shared_info struct refcnt field
> to refcnt_atomic.
>
> lib/librte_mbuf/rte_mbuf.c | 1 -
> lib/librte_mbuf/rte_mbuf.h | 19 ++++++++++---------
> lib/librte_mbuf/rte_mbuf_core.h | 11 +++--------
> 3 files changed, 13 insertions(+), 18 deletions(-)
>
> diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
> index ae91ae2..8a456e5 100644
> --- a/lib/librte_mbuf/rte_mbuf.c
> +++ b/lib/librte_mbuf/rte_mbuf.c
> @@ -22,7 +22,6 @@
> #include <rte_eal.h>
> #include <rte_per_lcore.h>
> #include <rte_lcore.h>
> -#include <rte_atomic.h>
> #include <rte_branch_prediction.h>
> #include <rte_mempool.h>
> #include <rte_mbuf.h>
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index f8e492e..4a7a98c 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -37,7 +37,6 @@
> #include <rte_config.h>
> #include <rte_mempool.h>
> #include <rte_memory.h>
> -#include <rte_atomic.h>
> #include <rte_prefetch.h>
> #include <rte_branch_prediction.h>
> #include <rte_byteorder.h>
> @@ -365,7 +364,7 @@ rte_pktmbuf_priv_flags(struct rte_mempool *mp)
> static inline uint16_t
> rte_mbuf_refcnt_read(const struct rte_mbuf *m)
> {
> - return (uint16_t)(rte_atomic16_read(&m->refcnt_atomic));
> + return __atomic_load_n(&m->refcnt, __ATOMIC_RELAXED);
> }
>
> /**
> @@ -378,14 +377,15 @@ rte_mbuf_refcnt_read(const struct rte_mbuf *m)
> static inline void
> rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
> {
> - rte_atomic16_set(&m->refcnt_atomic, (int16_t)new_value);
> + __atomic_store_n(&m->refcnt, new_value, __ATOMIC_RELAXED);
> }
>
> /* internal */
> static inline uint16_t
> __rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
> {
> - return (uint16_t)(rte_atomic16_add_return(&m->refcnt_atomic, value));
> + return (uint16_t)(__atomic_add_fetch((int16_t *)&m->refcnt, value,
> + __ATOMIC_ACQ_REL));
> }
>
> /**
> @@ -466,7 +466,7 @@ rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
> static inline uint16_t
> rte_mbuf_ext_refcnt_read(const struct rte_mbuf_ext_shared_info *shinfo)
> {
> - return (uint16_t)(rte_atomic16_read(&shinfo->refcnt_atomic));
> + return __atomic_load_n(&shinfo->refcnt_atomic, __ATOMIC_RELAXED);
> }
>
> /**
> @@ -481,7 +481,7 @@ static inline void
> rte_mbuf_ext_refcnt_set(struct rte_mbuf_ext_shared_info *shinfo,
> uint16_t new_value)
> {
> - rte_atomic16_set(&shinfo->refcnt_atomic, (int16_t)new_value);
> + __atomic_store_n(&shinfo->refcnt_atomic, new_value, __ATOMIC_RELAXED);
> }
>
> /**
> @@ -505,7 +505,8 @@ rte_mbuf_ext_refcnt_update(struct rte_mbuf_ext_shared_info *shinfo,
> return (uint16_t)value;
> }
>
> - return (uint16_t)rte_atomic16_add_return(&shinfo->refcnt_atomic, value);
> + return (uint16_t)(__atomic_add_fetch((int16_t *)&shinfo->refcnt_atomic,
> + value, __ATOMIC_ACQ_REL));
> }
>
> /** Mbuf prefetch */
> @@ -1304,8 +1305,8 @@ static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
> * Direct usage of add primitive to avoid
> * duplication of comparing with one.
> */
> - if (likely(rte_atomic16_add_return
> - (&shinfo->refcnt_atomic, -1)))
> + if (likely(__atomic_add_fetch((int *)&shinfo->refcnt_atomic, -1,
> + __ATOMIC_ACQ_REL)))
> return 1;
>
> /* Reinitialize counter before mbuf freeing. */
> diff --git a/lib/librte_mbuf/rte_mbuf_core.h b/lib/librte_mbuf/rte_mbuf_core.h
> index 16600f1..806313a 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -18,7 +18,6 @@
>
> #include <stdint.h>
> #include <rte_compat.h>
> -#include <generic/rte_atomic.h>
>
> #ifdef __cplusplus
> extern "C" {
> @@ -495,12 +494,8 @@ struct rte_mbuf {
> * or non-atomic) is controlled by the CONFIG_RTE_MBUF_REFCNT_ATOMIC
> * config option.
> */
> - RTE_STD_C11
> - union {
> - rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
> - /** Non-atomically accessed refcnt */
> - uint16_t refcnt;
> - };
> + uint16_t refcnt;
> +
It seems this patch does 2 things:
- remove refcnt_atomic
- use C11 atomics
The first change is an API break. I think it should be announced in a deprecation
notice. The one about atomic does not talk about it.
So I suggest to keep refcnt_atomic until next version.
> uint16_t nb_segs; /**< Number of segments. */
>
> /** Input port (16 bits to support more than 256 virtual ports).
> @@ -679,7 +674,7 @@ typedef void (*rte_mbuf_extbuf_free_callback_t)(void *addr, void *opaque);
> struct rte_mbuf_ext_shared_info {
> rte_mbuf_extbuf_free_callback_t free_cb; /**< Free callback function */
> void *fcb_opaque; /**< Free callback argument */
> - rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
> + uint16_t refcnt_atomic; /**< Atomically accessed refcnt */
> };
>
> /**< Maximum number of nb_segs allowed. */
> --
> 2.7.4
>
> -----Original Message-----
> From: Olivier Matz <olivier.matz@6wind.com>
> Sent: Wednesday, July 8, 2020 7:43 PM
> To: Phil Yang <Phil.Yang@arm.com>
> Cc: Stephen Hemminger <stephen@networkplumber.org>;
> david.marchand@redhat.com; dev@dpdk.org; drc@linux.vnet.ibm.com;
> Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt
> operations
>
> On Wed, Jul 08, 2020 at 04:48:33AM +0000, Phil Yang wrote:
> > > -----Original Message-----
> > > From: Stephen Hemminger <stephen@networkplumber.org>
> > > Sent: Wednesday, July 8, 2020 1:13 AM
> > > To: Phil Yang <Phil.Yang@arm.com>
> > > Cc: david.marchand@redhat.com; dev@dpdk.org;
> drc@linux.vnet.ibm.com;
> > > Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> > > olivier.matz@6wind.com; Ruifeng Wang <Ruifeng.Wang@arm.com>; nd
> > > <nd@arm.com>
> > > Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt
> > > operations
> > >
> > > On Tue, 7 Jul 2020 18:10:33 +0800
> > > Phil Yang <phil.yang@arm.com> wrote:
> > >
> > > > + return (uint16_t)(__atomic_add_fetch((int16_t *)&shinfo-
> > > >refcnt_atomic,
> > > > +
> > >
> > > Why do you need so many casts here?
> > > The type of refcnt_atomic is now uint16 after your patch.
> >
> > In the existing code, the input parameter type for this API is signed integer.
> For example:
> > drivers/net/netvsc/hn_rxtx.c:531
> > lib/librte_mbuf/rte_mbuf.h:1194
> >
> > However, the output type of rte_mbuf_ext_refcnt related APIs is not
> uniform. We use these typecast to consistent with the current API definition.
>
> Would it make sense to cast the increment instead?
Yes. It is better.
Thanks.
>
> I mean:
>
> return __atomic_add_fetch(&m->refcnt, (uint16_t)value,
> __ATOMIC_ACQ_REL);
>
> instead of:
>
> return (uint16_t)(__atomic_add_fetch((int16_t *)&m->refcnt, value,
> __ATOMIC_ACQ_REL));
>
>
> The same could apply to __rte_pktmbuf_pinned_extbuf_decref() I think:
>
> > - if (likely(rte_atomic16_add_return
> > - (&shinfo->refcnt_atomic, -1)))
> > + if (likely(__atomic_add_fetch((int *)&shinfo->refcnt_atomic, -1,
> > + __ATOMIC_ACQ_REL)))
>
> By the way, why was the cast was to (int *) in this case? I think
> it can overwrite fields beside refcnt.
Fixed in the next version.
Thanks,
Phil
> -----Original Message-----
> From: Olivier Matz <olivier.matz@6wind.com>
> Sent: Wednesday, July 8, 2020 7:44 PM
> To: Phil Yang <Phil.Yang@arm.com>
> Cc: david.marchand@redhat.com; dev@dpdk.org; drc@linux.vnet.ibm.com;
> Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v2] mbuf: use C11 atomics for refcnt operations
>
> Hi,
>
> On Tue, Jul 07, 2020 at 06:10:33PM +0800, Phil Yang wrote:
> > Use C11 atomics with explicit ordering instead of rte_atomic ops which
> > enforce unnecessary barriers on aarch64.
> >
> > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > ---
> > v2:
> > Fix ABI issue: revert the rte_mbuf_ext_shared_info struct refcnt field
> > to refcnt_atomic.
> >
> > lib/librte_mbuf/rte_mbuf.c | 1 -
> > lib/librte_mbuf/rte_mbuf.h | 19 ++++++++++---------
> > lib/librte_mbuf/rte_mbuf_core.h | 11 +++--------
> > 3 files changed, 13 insertions(+), 18 deletions(-)
> >
<snip>
>
> It seems this patch does 2 things:
> - remove refcnt_atomic
> - use C11 atomics
>
> The first change is an API break. I think it should be announced in a
> deprecation
> notice. The one about atomic does not talk about it.
>
> So I suggest to keep refcnt_atomic until next version.
Agreed.
I did a local test, this approach doesn't have any ABI breakage issue.
I will update in the next version.
Thanks,
Phil
>
>
> > uint16_t nb_segs; /**< Number of segments. */
> >
> > /** Input port (16 bits to support more than 256 virtual ports).
> > @@ -679,7 +674,7 @@ typedef void
> (*rte_mbuf_extbuf_free_callback_t)(void *addr, void *opaque);
> > struct rte_mbuf_ext_shared_info {
> > rte_mbuf_extbuf_free_callback_t free_cb; /**< Free callback
> function */
> > void *fcb_opaque; /**< Free callback argument */
> > - rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
> > + uint16_t refcnt_atomic; /**< Atomically accessed refcnt */
> > };
> >
> > /**< Maximum number of nb_segs allowed. */
> > --
> > 2.7.4
> >
@@ -22,7 +22,6 @@
#include <rte_eal.h>
#include <rte_per_lcore.h>
#include <rte_lcore.h>
-#include <rte_atomic.h>
#include <rte_branch_prediction.h>
#include <rte_mempool.h>
#include <rte_mbuf.h>
@@ -37,7 +37,6 @@
#include <rte_config.h>
#include <rte_mempool.h>
#include <rte_memory.h>
-#include <rte_atomic.h>
#include <rte_prefetch.h>
#include <rte_branch_prediction.h>
#include <rte_byteorder.h>
@@ -365,7 +364,7 @@ rte_pktmbuf_priv_flags(struct rte_mempool *mp)
static inline uint16_t
rte_mbuf_refcnt_read(const struct rte_mbuf *m)
{
- return (uint16_t)(rte_atomic16_read(&m->refcnt_atomic));
+ return __atomic_load_n(&m->refcnt, __ATOMIC_RELAXED);
}
/**
@@ -378,14 +377,15 @@ rte_mbuf_refcnt_read(const struct rte_mbuf *m)
static inline void
rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
{
- rte_atomic16_set(&m->refcnt_atomic, (int16_t)new_value);
+ __atomic_store_n(&m->refcnt, new_value, __ATOMIC_RELAXED);
}
/* internal */
static inline uint16_t
__rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
{
- return (uint16_t)(rte_atomic16_add_return(&m->refcnt_atomic, value));
+ return (uint16_t)(__atomic_add_fetch((int16_t *)&m->refcnt, value,
+ __ATOMIC_ACQ_REL));
}
/**
@@ -466,7 +466,7 @@ rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
static inline uint16_t
rte_mbuf_ext_refcnt_read(const struct rte_mbuf_ext_shared_info *shinfo)
{
- return (uint16_t)(rte_atomic16_read(&shinfo->refcnt_atomic));
+ return __atomic_load_n(&shinfo->refcnt_atomic, __ATOMIC_RELAXED);
}
/**
@@ -481,7 +481,7 @@ static inline void
rte_mbuf_ext_refcnt_set(struct rte_mbuf_ext_shared_info *shinfo,
uint16_t new_value)
{
- rte_atomic16_set(&shinfo->refcnt_atomic, (int16_t)new_value);
+ __atomic_store_n(&shinfo->refcnt_atomic, new_value, __ATOMIC_RELAXED);
}
/**
@@ -505,7 +505,8 @@ rte_mbuf_ext_refcnt_update(struct rte_mbuf_ext_shared_info *shinfo,
return (uint16_t)value;
}
- return (uint16_t)rte_atomic16_add_return(&shinfo->refcnt_atomic, value);
+ return (uint16_t)(__atomic_add_fetch((int16_t *)&shinfo->refcnt_atomic,
+ value, __ATOMIC_ACQ_REL));
}
/** Mbuf prefetch */
@@ -1304,8 +1305,8 @@ static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
* Direct usage of add primitive to avoid
* duplication of comparing with one.
*/
- if (likely(rte_atomic16_add_return
- (&shinfo->refcnt_atomic, -1)))
+ if (likely(__atomic_add_fetch((int *)&shinfo->refcnt_atomic, -1,
+ __ATOMIC_ACQ_REL)))
return 1;
/* Reinitialize counter before mbuf freeing. */
@@ -18,7 +18,6 @@
#include <stdint.h>
#include <rte_compat.h>
-#include <generic/rte_atomic.h>
#ifdef __cplusplus
extern "C" {
@@ -495,12 +494,8 @@ struct rte_mbuf {
* or non-atomic) is controlled by the CONFIG_RTE_MBUF_REFCNT_ATOMIC
* config option.
*/
- RTE_STD_C11
- union {
- rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
- /** Non-atomically accessed refcnt */
- uint16_t refcnt;
- };
+ uint16_t refcnt;
+
uint16_t nb_segs; /**< Number of segments. */
/** Input port (16 bits to support more than 256 virtual ports).
@@ -679,7 +674,7 @@ typedef void (*rte_mbuf_extbuf_free_callback_t)(void *addr, void *opaque);
struct rte_mbuf_ext_shared_info {
rte_mbuf_extbuf_free_callback_t free_cb; /**< Free callback function */
void *fcb_opaque; /**< Free callback argument */
- rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
+ uint16_t refcnt_atomic; /**< Atomically accessed refcnt */
};
/**< Maximum number of nb_segs allowed. */