[{"id":3677300,"web_url":"http://patchwork.ozlabs.org/comment/3677300/","msgid":"<875x5tzfmn.fsf@suse.de>","list_archive_url":null,"date":"2026-04-14T16:11:12","subject":"Re: [PATCH v5] migration/rdma: add x-rdma-chunk-size parameter","submitter":{"id":85343,"url":"http://patchwork.ozlabs.org/api/people/85343/","name":"Fabiano Rosas","email":"farosas@suse.de"},"content":"Samuel Zhang writes:\n\n> The default 1MB RDMA chunk size causes slow live migration because\n> each chunk triggers a write_flush (ibv_post_send). For 8GB RAM,\n> 1MB chunk size produces ~15000 flushes vs ~3700 with 1024MB chunk size.\n>\n> Add x-rdma-chunk-size parameter to configure the RDMA chunk size for\n> faster migration.\n> Usage: `migrate_set_parameter x-rdma-chunk-size 1024M`\n>\n> Performance with RDMA live migration of 8GB RAM VM:\n>\n> | x-rdma-chunk-size (B) | time (s) | throughput (MB/s) |\n> |-----------------------|----------|-------------------|\n> | 1M (default) | 37.915 | 1,007 |\n> | 32M | 17.880 | 2,260 |\n> | 1024M | 4.368 | 17,529 |\n>\n> Signed-off-by: Samuel Zhang \n> Acked-by: Markus Armbruster \n> Acked-by: Li Zhijian \n> Tested-by: Li Zhijian \n> ---\n> v2:\n> - Renamed x-rdma-chunk-shift to x-rdma-chunk-size (byte count)\n> - Added validation in migrate_params_check()\n> - Added hmp_migrate_set_parameter() support\n> - Added hmp_info_migrate_parameters() support\n> - Added migrate_mark_all_params_present()\n> - Use qemu_strtosz() for size suffix support\n> v3: [Markus]\n> - Use visit_type_size() in HMP set parameter\n> - Use MiB/GiB constants\n> v4: [Markus]\n> - Remove superfluous comment on DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE\n> - Use \"Only applies when migrating via RDMA\" in QAPI doc\n> v5:\n> - Document that x-rdma-chunk-size must be set to the same value on both\n> source and destination before migration starts.\n> - Add Acked-by and Tested-by from Li Zhijian.\n>\n> migration/migration-hmp-cmds.c | 11 +++++++++++\n> migration/options.c | 33 ++++++++++++++++++++++++++++++++-\n> migration/options.h | 1 +\n> migration/rdma.c | 30 ++++++++++++++++--------------\n> qapi/migration.json | 13 +++++++++++--\n> 5 files changed, 71 insertions(+), 17 deletions(-)\n>\n> diff --git a/migration/migration-hmp-cmds.c b/migration/migration-hmp-cmds.c\n> index 0a193b8f54..4f6c1dbf89 100644\n> --- a/migration/migration-hmp-cmds.c\n> +++ b/migration/migration-hmp-cmds.c\n> @@ -451,6 +451,13 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict)\n> params->direct_io ? \"on\" : \"off\");\n> }\n> \n> + if (params->has_x_rdma_chunk_size) {\n> + monitor_printf(mon, \"%s: %\" PRIu64 \" bytes\\n\",\n> + MigrationParameter_str(\n> + MIGRATION_PARAMETER_X_RDMA_CHUNK_SIZE),\n> + params->x_rdma_chunk_size);\n> + }\n> +\n> assert(params->has_cpr_exec_command);\n> monitor_print_cpr_exec_command(mon, params->cpr_exec_command);\n> }\n> @@ -734,6 +741,10 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict *qdict)\n> p->has_direct_io = true;\n> visit_type_bool(v, param, &p->direct_io, &err);\n> break;\n> + case MIGRATION_PARAMETER_X_RDMA_CHUNK_SIZE:\n> + p->has_x_rdma_chunk_size = true;\n> + visit_type_size(v, param, &p->x_rdma_chunk_size, &err);\n> + break;\n> case MIGRATION_PARAMETER_CPR_EXEC_COMMAND: {\n> /*\n> * NOTE: g_autofree will only auto g_free() the strv array when\n> diff --git a/migration/options.c b/migration/options.c\n> index 7556fbc06b..2b5158200b 100644\n> --- a/migration/options.c\n> +++ b/migration/options.c\n> @@ -13,6 +13,7 @@\n> \n> #include \"qemu/osdep.h\"\n> #include \"qemu/error-report.h\"\n> +#include \"qemu/units.h\"\n> #include \"exec/target_page.h\"\n> #include \"qapi/clone-visitor.h\"\n> #include \"qapi/error.h\"\n> @@ -90,6 +91,7 @@ const PropertyInfo qdev_prop_StrOrNull;\n> \n> #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT_PERIOD 1000 /* milliseconds */\n> #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT 1 /* MB/s */\n> +#define DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE MiB\n> \n> const Property migration_properties[] = {\n> DEFINE_PROP_BOOL(\"store-global-state\", MigrationState,\n> @@ -183,6 +185,9 @@ const Property migration_properties[] = {\n> DEFINE_PROP_ZERO_PAGE_DETECTION(\"zero-page-detection\", MigrationState,\n> parameters.zero_page_detection,\n> ZERO_PAGE_DETECTION_MULTIFD),\n> + DEFINE_PROP_UINT64(\"x-rdma-chunk-size\", MigrationState,\n> + parameters.x_rdma_chunk_size,\n> + DEFAULT_MIGRATE_X_RDMA_CHUNK_SIZE),\n> \n> /* Migration capabilities */\n> DEFINE_PROP_MIG_CAP(\"x-xbzrle\", MIGRATION_CAPABILITY_XBZRLE),\n> @@ -1000,6 +1005,15 @@ ZeroPageDetection migrate_zero_page_detection(void)\n> return s->parameters.zero_page_detection;\n> }\n> \n> +uint64_t migrate_rdma_chunk_size(void)\n> +{\n> + MigrationState *s = migrate_get_current();\n> + uint64_t size = s->parameters.x_rdma_chunk_size;\n> +\n> + assert(MiB <= size && size <= GiB && is_power_of_2(size));\n> + return size;\n> +}\n> +\n> /* parameters helpers */\n> \n> AnnounceParameters *migrate_announce_params(void)\n> @@ -1062,7 +1076,7 @@ static void migrate_mark_all_params_present(MigrationParameters *p)\n> &p->has_announce_step, &p->has_block_bitmap_mapping,\n> &p->has_x_vcpu_dirty_limit_period, &p->has_vcpu_dirty_limit,\n> &p->has_mode, &p->has_zero_page_detection, &p->has_direct_io,\n> - &p->has_cpr_exec_command,\n> + &p->has_x_rdma_chunk_size, &p->has_cpr_exec_command,\n> };\n> \n> len = ARRAY_SIZE(has_fields);\n> @@ -1273,6 +1287,15 @@ bool migrate_params_check(MigrationParameters *params, Error **errp)\n> return false;\n> }\n> \n> + if (params->has_x_rdma_chunk_size &&\n> + (params->x_rdma_chunk_size < MiB ||\n> + params->x_rdma_chunk_size > GiB ||\n> + !is_power_of_2(params->x_rdma_chunk_size))) {\n> + error_setg(errp, \"Option x_rdma_chunk_size expects \"\n> + \"a power of 2 in the range 1MiB to 1024MiB\");\n> + return false;\n> + }\n> +\n> return true;\n> }\n> \n> @@ -1398,6 +1421,10 @@ static void migrate_params_test_apply(MigrationParameters *params,\n> dest->direct_io = params->direct_io;\n> }\n> \n> + if (params->has_x_rdma_chunk_size) {\n> + dest->x_rdma_chunk_size = params->x_rdma_chunk_size;\n> + }\n> +\n> if (params->has_cpr_exec_command) {\n> dest->cpr_exec_command = params->cpr_exec_command;\n> }\n> @@ -1524,6 +1551,10 @@ static void migrate_params_apply(MigrationParameters *params)\n> s->parameters.direct_io = params->direct_io;\n> }\n> \n> + if (params->has_x_rdma_chunk_size) {\n> + s->parameters.x_rdma_chunk_size = params->x_rdma_chunk_size;\n> + }\n> +\n> if (params->has_cpr_exec_command) {\n> qapi_free_strList(s->parameters.cpr_exec_command);\n> s->parameters.cpr_exec_command =\n> diff --git a/migration/options.h b/migration/options.h\n> index b502871097..b46221998a 100644\n> --- a/migration/options.h\n> +++ b/migration/options.h\n> @@ -87,6 +87,7 @@ const char *migrate_tls_creds(void);\n> const char *migrate_tls_hostname(void);\n> uint64_t migrate_xbzrle_cache_size(void);\n> ZeroPageDetection migrate_zero_page_detection(void);\n> +uint64_t migrate_rdma_chunk_size(void);\n> \n> /* parameters helpers */\n> \n> diff --git a/migration/rdma.c b/migration/rdma.c\n> index 55ab85650a..3e37a1d440 100644\n> --- a/migration/rdma.c\n> +++ b/migration/rdma.c\n> @@ -45,10 +45,12 @@\n> #define RDMA_RESOLVE_TIMEOUT_MS 10000\n> \n> /* Do not merge data if larger than this. */\n> -#define RDMA_MERGE_MAX (2 * 1024 * 1024)\n> -#define RDMA_SIGNALED_SEND_MAX (RDMA_MERGE_MAX / 4096)\n> +static inline uint64_t rdma_merge_max(void)\n> +{\n> + return migrate_rdma_chunk_size() * 2;\n> +}\n> \n> -#define RDMA_REG_CHUNK_SHIFT 20 /* 1 MB */\n> +#define RDMA_SIGNALED_SEND_MAX 512\n> \n> /*\n> * This is only for non-live state being migrated.\n> @@ -527,21 +529,21 @@ static int qemu_rdma_exchange_send(RDMAContext *rdma, RDMAControlHeader *head,\n> static inline uint64_t ram_chunk_index(const uint8_t *start,\n> const uint8_t *host)\n> {\n> - return ((uintptr_t) host - (uintptr_t) start) >> RDMA_REG_CHUNK_SHIFT;\n> + return ((uintptr_t) host - (uintptr_t) start) / migrate_rdma_chunk_size();\n> }\n> \n> static inline uint8_t *ram_chunk_start(const RDMALocalBlock *rdma_ram_block,\n> uint64_t i)\n> {\n> return (uint8_t *)(uintptr_t)(rdma_ram_block->local_host_addr +\n> - (i << RDMA_REG_CHUNK_SHIFT));\n> + (i * migrate_rdma_chunk_size()));\n> }\n> \n> static inline uint8_t *ram_chunk_end(const RDMALocalBlock *rdma_ram_block,\n> uint64_t i)\n> {\n> uint8_t *result = ram_chunk_start(rdma_ram_block, i) +\n> - (1UL << RDMA_REG_CHUNK_SHIFT);\n> + migrate_rdma_chunk_size();\n> \n> if (result > (rdma_ram_block->local_host_addr + rdma_ram_block->length)) {\n> result = rdma_ram_block->local_host_addr + rdma_ram_block->length;\n> @@ -1841,6 +1843,7 @@ static int qemu_rdma_write_one(RDMAContext *rdma,\n> struct ibv_send_wr *bad_wr;\n> int reg_result_idx, ret, count = 0;\n> uint64_t chunk, chunks;\n> + uint64_t chunk_size = migrate_rdma_chunk_size();\n> uint8_t *chunk_start, *chunk_end;\n> RDMALocalBlock *block = &(rdma->local_ram_blocks.block[current_index]);\n> RDMARegister reg;\n> @@ -1861,22 +1864,21 @@ retry:\n> chunk_start = ram_chunk_start(block, chunk);\n> \n> if (block->is_ram_block) {\n> - chunks = length / (1UL << RDMA_REG_CHUNK_SHIFT);\n> + chunks = length / chunk_size;\n> \n> - if (chunks && ((length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {\n> + if (chunks && ((length % chunk_size) == 0)) {\n> chunks--;\n> }\n> } else {\n> - chunks = block->length / (1UL << RDMA_REG_CHUNK_SHIFT);\n> + chunks = block->length / chunk_size;\n> \n> - if (chunks && ((block->length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {\n> + if (chunks && ((block->length % chunk_size) == 0)) {\n> chunks--;\n> }\n> }\n> \n> trace_qemu_rdma_write_one_top(chunks + 1,\n> - (chunks + 1) *\n> - (1UL << RDMA_REG_CHUNK_SHIFT) / 1024 / 1024);\n> + (chunks + 1) * chunk_size / 1024 / 1024);\n> \n> chunk_end = ram_chunk_end(block, chunk + chunks);\n> \n> @@ -2176,7 +2178,7 @@ static int qemu_rdma_write(RDMAContext *rdma,\n> rdma->current_length += len;\n> \n> /* flush it if buffer is too large */\n> - if (rdma->current_length >= RDMA_MERGE_MAX) {\n> + if (rdma->current_length >= rdma_merge_max()) {\n> return qemu_rdma_write_flush(rdma, errp);\n> }\n> \n> @@ -3522,7 +3524,7 @@ int rdma_registration_handle(QEMUFile *f)\n> } else {\n> chunk = reg->key.chunk;\n> host_addr = block->local_host_addr +\n> - (reg->key.chunk * (1UL << RDMA_REG_CHUNK_SHIFT));\n> + (reg->key.chunk * migrate_rdma_chunk_size());\n> /* Check for particularly bad chunk value */\n> if (host_addr < (void *)block->local_host_addr) {\n> error_report(\"rdma: bad chunk for block %s\"\n> diff --git a/qapi/migration.json b/qapi/migration.json\n> index 7134d4ce47..0db115ec5e 100644\n> --- a/qapi/migration.json\n> +++ b/qapi/migration.json\n> @@ -806,7 +806,7 @@\n> #\n> # Features:\n> #\n> -# @unstable: Members @x-checkpoint-delay and\n> +# @unstable: Members @x-checkpoint-delay, @x-rdma-chunk-size, and\n> # @x-vcpu-dirty-limit-period are experimental.\n> #\n> # Since: 2.4\n> @@ -831,6 +831,7 @@\n> 'mode',\n> 'zero-page-detection',\n> 'direct-io',\n> + { 'name': 'x-rdma-chunk-size', 'features': [ 'unstable' ] },\n> 'cpr-exec-command'] }\n> \n> ##\n> @@ -1007,9 +1008,15 @@\n> # is @cpr-exec. The first list element is the program's filename,\n> # the remainder its arguments. (Since 10.2)\n> #\n> +# @x-rdma-chunk-size: RDMA memory registration chunk size in bytes.\n> +# Default is 1MiB. Must be a power of 2 in the range\n> +# [1MiB, 1024MiB]. Only applies when migrating via RDMA.\n> +# Must be set to the same value on both source and destination\n> +# before migration starts. (Since 11.1)\n> +#\n> # Features:\n> #\n> -# @unstable: Members @x-checkpoint-delay and\n> +# @unstable: Members @x-checkpoint-delay, @x-rdma-chunk-size, and\n> # @x-vcpu-dirty-limit-period are experimental.\n> #\n> # Since: 2.4\n> @@ -1046,6 +1053,8 @@\n> '*mode': 'MigMode',\n> '*zero-page-detection': 'ZeroPageDetection',\n> '*direct-io': 'bool',\n> + '*x-rdma-chunk-size': { 'type': 'uint64',\n> + 'features': [ 'unstable' ] },\n> '*cpr-exec-command': [ 'str' ]} }\n> \n> ##\n\nAcked-by: Fabiano Rosas ","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=suse.de header.i=@suse.de header.a=rsa-sha256\n header.s=susede2_rsa header.b=BlxPt5gO;\n\tdkim=pass header.d=suse.de header.i=@suse.de header.a=ed25519-sha256\n header.s=susede2_ed25519 header.b=CI2Iq48u;\n\tdkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de\n header.a=rsa-sha256 header.s=susede2_rsa header.b=BlxPt5gO;\n\tdkim=neutral header.d=suse.de header.i=@suse.de header.a=ed25519-sha256\n header.s=susede2_ed25519 header.b=CI2Iq48u;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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a=ed25519-sha256; c=relaxed/relaxed; d=suse.de;\n s=susede2_ed25519; t=1776183079;\n h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc:\n mime-version:mime-version:content-type:content-type:\n in-reply-to:in-reply-to:references:references;\n bh=xWSVVoxSb5wBAmySmkuBOM4XrDfX0XFdBiMFTttefAw=;\n b=CI2Iq48u5nhSo6Qnfcmx9wKDkPaxiaj/2IUp8SFg0sTcONybOFwZ2EcJsXjoJbTfUPsGT8\n rcOhz6IxFxb6EWAg=="],"From":"Fabiano Rosas ","To":"Samuel Zhang , qemu-devel@nongnu.org","Cc":"peterx@redhat.com, lizhijian@fujitsu.com, eblake@redhat.com,\n armbru@redhat.com, Emily.Deng@amd.com, Victor.Zhao@amd.com,\n PengJu.Zhou@amd.com, Qing.Ma@amd.com, Samuel Zhang ","Subject":"Re: [PATCH v5] migration/rdma: add x-rdma-chunk-size parameter","In-Reply-To":"<20260413035703.2602065-1-guoqing.zhang@amd.com>","References":"<20260413035703.2602065-1-guoqing.zhang@amd.com>","Date":"Tue, 14 Apr 2026 13:11:12 -0300","Message-ID":"<875x5tzfmn.fsf@suse.de>","MIME-Version":"1.0","Content-Type":"text/plain","X-Rspamd-Action":"no action","X-Rspamd-Server":"rspamd2.dmz-prg2.suse.org","X-Spamd-Result":"default: False [-4.51 / 50.00]; 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For 8GB RAM,\n> 1MB chunk size produces ~15000 flushes vs ~3700 with 1024MB chunk size.\n> \n> Add x-rdma-chunk-size parameter to configure the RDMA chunk size for\n> faster migration.\n> Usage: `migrate_set_parameter x-rdma-chunk-size 1024M`\n> \n> Performance with RDMA live migration of 8GB RAM VM:\n> \n> | x-rdma-chunk-size (B) | time (s) | throughput (MB/s) |\n> |-----------------------|----------|-------------------|\n> | 1M (default) | 37.915 | 1,007 |\n> | 32M | 17.880 | 2,260 |\n> | 1024M | 4.368 | 17,529 |\n> \n> Signed-off-by: Samuel Zhang \n> Acked-by: Markus Armbruster \n> Acked-by: Li Zhijian \n> Tested-by: Li Zhijian \n\nAcked-by: Peter Xu ","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=B1yFxKr6;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=google header.b=S5CCXkeq;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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