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migration/rdma: add x-rdma-chunk-size parameter

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The default 1MB RDMA chunk size causes slow live migration because
each chunk triggers a write_flush (ibv_post_send). For 8GB RAM,
1MB chunk size produces ~15000 flushes vs ~3700 with 1024MB chunk size.

Add x-rdma-chunk-size parameter to configure the RDMA chunk size for
faster migration.
Usage: `migrate_set_parameter x-rdma-chunk-size 1024M`

Performance with RDMA live migration of 8GB RAM VM:

| x-rdma-chunk-size (B) | time (s) | throughput (MB/s) |
|-----------------------|----------|-------------------|
| 1M (default)          | 37.915   |  1,007            |
| 32M                   | 17.880   |  2,260            |
| 1024M                 |  4.368   | 17,529            |

Signed-off-by: Samuel Zhang <guoqing.zhang@amd.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Li Zhijian <lizhijian@fujitsu.com>
Tested-by: Li Zhijian <lizhijian@fujitsu.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20260427031401.3895523-1-guoqing.zhang@amd.com
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Samuel Zhang
2026-04-27 11:14:01 +08:00
committed by Peter Xu
parent f77a7cec9f
commit 97dc5cf045
5 changed files with 71 additions and 17 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
migration/migration-hmp-cmds.c
View File

@@ -451,6 +451,13 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict)
params->direct_io ? "on" : "off");
}
if (params->has_x_rdma_chunk_size) {
monitor_printf(mon, "%s: %" PRIu64 " bytes\n",
MigrationParameter_str(
MIGRATION_PARAMETER_X_RDMA_CHUNK_SIZE),
params->x_rdma_chunk_size);
}
assert(params->has_cpr_exec_command);
monitor_print_cpr_exec_command(mon, params->cpr_exec_command);
}
@@ -734,6 +741,10 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict *qdict)
p->has_direct_io = true;
visit_type_bool(v, param, &p->direct_io, &err);
break;
case MIGRATION_PARAMETER_X_RDMA_CHUNK_SIZE:
p->has_x_rdma_chunk_size = true;
visit_type_size(v, param, &p->x_rdma_chunk_size, &err);
break;
case MIGRATION_PARAMETER_CPR_EXEC_COMMAND: {
/*
* NOTE: g_autofree will only auto g_free() the strv array when

33
migration/options.c
View File

@@ -13,6 +13,7 @@
#include "qemu/osdep.h"
#include "qemu/error-report.h"
#include "qemu/units.h"
#include "exec/target_page.h"
#include "qapi/clone-visitor.h"
#include "qapi/error.h"
@@ -90,6 +91,7 @@ const PropertyInfo qdev_prop_StrOrNull;
#define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT_PERIOD 1000 /* milliseconds */
#define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT 1 /* MB/s */
#define DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE MiB
const Property migration_properties[] = {
DEFINE_PROP_BOOL("store-global-state", MigrationState,
@@ -183,6 +185,9 @@ const Property migration_properties[] = {
DEFINE_PROP_ZERO_PAGE_DETECTION("zero-page-detection", MigrationState,
parameters.zero_page_detection,
ZERO_PAGE_DETECTION_MULTIFD),
DEFINE_PROP_UINT64("x-rdma-chunk-size", MigrationState,
parameters.x_rdma_chunk_size,
DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE),
/* Migration capabilities */
DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
@@ -1000,6 +1005,15 @@ ZeroPageDetection migrate_zero_page_detection(void)
return s->parameters.zero_page_detection;
}
uint64_t migrate_rdma_chunk_size(void)
{
MigrationState *s = migrate_get_current();
uint64_t size = s->parameters.x_rdma_chunk_size;
assert(MiB <= size && size <= GiB && is_power_of_2(size));
return size;
}
/* parameters helpers */
AnnounceParameters *migrate_announce_params(void)
@@ -1062,7 +1076,7 @@ static void migrate_mark_all_params_present(MigrationParameters *p)
&p->has_announce_step, &p->has_block_bitmap_mapping,
&p->has_x_vcpu_dirty_limit_period, &p->has_vcpu_dirty_limit,
&p->has_mode, &p->has_zero_page_detection, &p->has_direct_io,
&p->has_cpr_exec_command,
&p->has_x_rdma_chunk_size, &p->has_cpr_exec_command,
};
len = ARRAY_SIZE(has_fields);
@@ -1273,6 +1287,15 @@ bool migrate_params_check(MigrationParameters *params, Error **errp)
return false;
}
if (params->has_x_rdma_chunk_size &&
(params->x_rdma_chunk_size < MiB ||
params->x_rdma_chunk_size > GiB ||
!is_power_of_2(params->x_rdma_chunk_size))) {
error_setg(errp, "Option x_rdma_chunk_size expects "
"a power of 2 in the range 1MiB to 1024MiB");
return false;
}
return true;
}
@@ -1393,6 +1416,10 @@ static void migrate_params_test_apply(MigrationParameters *params,
dest->direct_io = params->direct_io;
}
if (params->has_x_rdma_chunk_size) {
dest->x_rdma_chunk_size = params->x_rdma_chunk_size;
}
if (params->has_cpr_exec_command) {
qapi_free_strList(dest->cpr_exec_command);
dest->cpr_exec_command = QAPI_CLONE(strList, params->cpr_exec_command);
@@ -1520,6 +1547,10 @@ static void migrate_params_apply(MigrationParameters *params)
s->parameters.direct_io = params->direct_io;
}
if (params->has_x_rdma_chunk_size) {
s->parameters.x_rdma_chunk_size = params->x_rdma_chunk_size;
}
if (params->has_cpr_exec_command) {
qapi_free_strList(s->parameters.cpr_exec_command);
s->parameters.cpr_exec_command =

1
migration/options.h
View File

@@ -87,6 +87,7 @@ const char *migrate_tls_creds(void);
const char *migrate_tls_hostname(void);
uint64_t migrate_xbzrle_cache_size(void);
ZeroPageDetection migrate_zero_page_detection(void);
uint64_t migrate_rdma_chunk_size(void);
/* parameters helpers */

30
migration/rdma.c
View File

@@ -45,10 +45,12 @@
#define RDMA_RESOLVE_TIMEOUT_MS 10000
/* Do not merge data if larger than this. */
#define RDMA_MERGE_MAX (2 * 1024 * 1024)
#define RDMA_SIGNALED_SEND_MAX (RDMA_MERGE_MAX / 4096)
static inline uint64_t rdma_merge_max(void)
{
return migrate_rdma_chunk_size() * 2;
}
#define RDMA_REG_CHUNK_SHIFT 20 /* 1 MB */
#define RDMA_SIGNALED_SEND_MAX 512
/*
* This is only for non-live state being migrated.
@@ -527,21 +529,21 @@ static int qemu_rdma_exchange_send(RDMAContext *rdma, RDMAControlHeader *head,
static inline uint64_t ram_chunk_index(const uint8_t *start,
const uint8_t *host)
{
return ((uintptr_t) host - (uintptr_t) start) >> RDMA_REG_CHUNK_SHIFT;
return ((uintptr_t) host - (uintptr_t) start) / migrate_rdma_chunk_size();
}
static inline uint8_t *ram_chunk_start(const RDMALocalBlock *rdma_ram_block,
uint64_t i)
{
return (uint8_t *)(uintptr_t)(rdma_ram_block->local_host_addr +
(i << RDMA_REG_CHUNK_SHIFT));
(i * migrate_rdma_chunk_size()));
}
static inline uint8_t *ram_chunk_end(const RDMALocalBlock *rdma_ram_block,
uint64_t i)
{
uint8_t *result = ram_chunk_start(rdma_ram_block, i) +
(1UL << RDMA_REG_CHUNK_SHIFT);
migrate_rdma_chunk_size();
if (result > (rdma_ram_block->local_host_addr + rdma_ram_block->length)) {
result = rdma_ram_block->local_host_addr + rdma_ram_block->length;
@@ -1841,6 +1843,7 @@ static int qemu_rdma_write_one(RDMAContext *rdma,
struct ibv_send_wr *bad_wr;
int reg_result_idx, ret, count = 0;
uint64_t chunk, chunks;
uint64_t chunk_size = migrate_rdma_chunk_size();
uint8_t *chunk_start, *chunk_end;
RDMALocalBlock *block = &(rdma->local_ram_blocks.block[current_index]);
RDMARegister reg;
@@ -1861,22 +1864,21 @@ retry:
chunk_start = ram_chunk_start(block, chunk);
if (block->is_ram_block) {
chunks = length / (1UL << RDMA_REG_CHUNK_SHIFT);
chunks = length / chunk_size;
if (chunks && ((length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {
if (chunks && ((length % chunk_size) == 0)) {
chunks--;
}
} else {
chunks = block->length / (1UL << RDMA_REG_CHUNK_SHIFT);
chunks = block->length / chunk_size;
if (chunks && ((block->length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {
if (chunks && ((block->length % chunk_size) == 0)) {
chunks--;
}
}
trace_qemu_rdma_write_one_top(chunks + 1,
(chunks + 1) *
(1UL << RDMA_REG_CHUNK_SHIFT) / 1024 / 1024);
(chunks + 1) * chunk_size / 1024 / 1024);
chunk_end = ram_chunk_end(block, chunk + chunks);
@@ -2176,7 +2178,7 @@ static int qemu_rdma_write(RDMAContext *rdma,
rdma->current_length += len;
/* flush it if buffer is too large */
if (rdma->current_length >= RDMA_MERGE_MAX) {
if (rdma->current_length >= rdma_merge_max()) {
return qemu_rdma_write_flush(rdma, errp);
}
@@ -3522,7 +3524,7 @@ int rdma_registration_handle(QEMUFile *f)
} else {
chunk = reg->key.chunk;
host_addr = block->local_host_addr +
(reg->key.chunk * (1UL << RDMA_REG_CHUNK_SHIFT));
(reg->key.chunk * migrate_rdma_chunk_size());
/* Check for particularly bad chunk value */
if (host_addr < (void *)block->local_host_addr) {
error_report("rdma: bad chunk for block %s"

13
qapi/migration.json
View File

@@ -806,7 +806,7 @@
#
# Features:
#
# @unstable: Members @x-checkpoint-delay and
# @unstable: Members @x-checkpoint-delay, @x-rdma-chunk-size, and
# @x-vcpu-dirty-limit-period are experimental.
#
# Since: 2.4
@@ -831,6 +831,7 @@
'mode',
'zero-page-detection',
'direct-io',
{ 'name': 'x-rdma-chunk-size', 'features': [ 'unstable' ] },
'cpr-exec-command'] }
##
@@ -1007,9 +1008,15 @@
# is @cpr-exec. The first list element is the program's filename,
# the remainder its arguments. (Since 10.2)
#
# @x-rdma-chunk-size: RDMA memory registration chunk size in bytes.
# Default is 1MiB. Must be a power of 2 in the range
# [1MiB, 1024MiB]. Only applies when migrating via RDMA.
# Must be set to the same value on both source and destination
# before migration starts. (Since 11.1)
#
# Features:
#
# @unstable: Members @x-checkpoint-delay and
# @unstable: Members @x-checkpoint-delay, @x-rdma-chunk-size, and
# @x-vcpu-dirty-limit-period are experimental.
#
# Since: 2.4
@@ -1046,6 +1053,8 @@
'*mode': 'MigMode',
'*zero-page-detection': 'ZeroPageDetection',
'*direct-io': 'bool',
'*x-rdma-chunk-size': { 'type': 'uint64',
'features': [ 'unstable' ] },
'*cpr-exec-command': [ 'str' ]} }
##