Re: [PATCH v3 04/10] multifd: Add COLO support

[Lukas Straub <lukasstraub2@web.de>]

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On Mon, 26 Jan 2026 11:33:13 -0300
Fabiano Rosas <farosas@suse.de> wrote:

> Lukas Straub <lukasstraub2@web.de> writes:
> 
> > Like in the normal ram_load() path, put the received pages into the
> > colo cache and mark the pages in the bitmap so that they will be
> > flushed to the guest later.
> >  
> 
> 
> 
> > Multifd with COLO is useful to reduce the VM pause time during checkpointing
> > for latency sensitive workloads. In such workloads the worst-case latency
> > is especially important.
> >
> > Also, this is already worth it for the precopy phase as it helps with
> > converging. Moreover, multifd migration is the preferred way to do migration
> > nowadays and this allows to use multifd compression with COLO.
> >
> > Benchmark:
> > Cluster nodes
> >  - Intel Xenon E5-2630 v3
> >  - 48Gb RAM
> >  - 10G Ethernet
> > Guest
> >  - Windows Server 2016
> >  - 6Gb RAM
> >  - 4 cores
> > Workload
> >  - Upload a file to the guest with SMB to simulate moderate
> >    memory dirtying
> >  - Measure the memory transfer time portion of each checkpoint
> >  - 600ms COLO checkpoint interval
> >
> > Results
> > Plain
> >  idle mean: 4.50ms 99per: 10.33ms
> >  load mean: 24.30ms 99per: 78.05ms
> > Multifd-4
> >  idle mean: 6.48ms 99per: 10.41ms
> >  load mean: 14.12ms 99per: 31.27ms
> >
> > Evaluation
> > While multifd has slightly higher latency when the guest idles, it is
> > 10ms faster under load and more importantly it's worst case latency is
> > less than 1/2 of plain under load as can be seen in the 99. Percentile.
> >
> > Signed-off-by: Juan Quintela <quintela@redhat.com>
> > Signed-off-by: Lukas Straub <lukasstraub2@web.de>
> > ---
> >  MAINTAINERS                |  1 +
> >  migration/meson.build      |  2 +-
> >  migration/multifd-colo.c   | 50 ++++++++++++++++++++++++++++++++++++++++++++++
> >  migration/multifd-colo.h   | 26 ++++++++++++++++++++++++
> >  migration/multifd-nocomp.c | 10 +++++++++-
> >  migration/multifd.c        |  8 ++++++++
> >  migration/multifd.h        |  5 ++++-
> >  7 files changed, 99 insertions(+), 3 deletions(-)
> >
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 1e9bdd87c3a2f84f3abfc56986cd793976810fdd..883f0a8f4eb92d0bf0f89fcab4674ccc4aed1cc1 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -3853,6 +3853,7 @@ COLO Framework
> >  M: Lukas Straub <lukasstraub2@web.de>
> >  S: Maintained
> >  F: migration/colo*
> > +F: migration/multifd-colo.*
> >  F: include/migration/colo.h
> >  F: include/migration/failover.h
> >  F: docs/COLO-FT.txt
> > diff --git a/migration/meson.build b/migration/meson.build
> > index c7f39bdb55239ecb0e775c77b90a1aa9e6a4a9ce..c9f0f5f9f2137536497e53e960ce70654ad1b394 100644
> > --- a/migration/meson.build
> > +++ b/migration/meson.build
> > @@ -39,7 +39,7 @@ system_ss.add(files(
> >  ), gnutls, zlib)
> >  
> >  if get_option('replication').allowed()
> > -  system_ss.add(files('colo-failover.c', 'colo.c'))
> > +  system_ss.add(files('colo-failover.c', 'colo.c', 'multifd-colo.c'))
> >  else
> >    system_ss.add(files('colo-stubs.c'))
> >  endif
> > diff --git a/migration/multifd-colo.c b/migration/multifd-colo.c
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..c47f5044663969e0c9af56da5ec34902d635810a
> > --- /dev/null
> > +++ b/migration/multifd-colo.c
> > @@ -0,0 +1,50 @@
> > +/*
> > + * SPDX-License-Identifier: GPL-2.0-or-later
> > + *
> > + * multifd colo implementation
> > + *
> > + * Copyright (c) Lukas Straub <lukasstraub2@web.de>
> > + *
> > + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> > + * See the COPYING file in the top-level directory.
> > + */
> > +
> > +#include "qemu/osdep.h"
> > +#include "exec/target_page.h"
> > +#include "qemu/error-report.h"
> > +#include "qapi/error.h"
> > +#include "ram.h"
> > +#include "multifd.h"
> > +#include "options.h"
> > +#include "io/channel-socket.h"
> > +#include "migration/colo.h"
> > +#include "multifd-colo.h"
> > +#include "system/ramblock.h"
> > +
> > +void multifd_colo_prepare_recv(MultiFDRecvParams *p)
> > +{
> > +    /*
> > +     * While we're still in precopy state (not yet in colo state), we copy
> > +     * received pages to both guest and cache. No need to set dirty bits,
> > +     * since guest and cache memory are in sync.
> > +     */
> > +    if (migration_incoming_in_colo_state()) {  
> 
> What's the relationship between migration_incoming_in_colo_state() and
> migration_incoming_colo_enabled()? ram_load_precopy() checks both. Would
> migration_incoming_colo_enabled affect multifd as well?

So first off migration_incoming_colo_enabled() and migrate_colo()
are equivalent in practice since
121ccedc2b migration: block incoming colo when capability is disabled

(I have some cleanup patches lying around, but that will be for later)

For colo, we do normal precopy migration and at the end we go into colo
state and then
migration_incoming_in_colo_state() will be true.

Here we check migrate_colo() outside of these functions as Peter
requested that in a previous version.

> 
> The multifd recv threads will be running until after
> process_incoming_migration_bh(), which is when
> migration_incoming_disable_colo() runs.

That is not an issue as we use migrate_colo() here.

> 
> Also, is the colo_cache guaranteed to be around until multifd threads
> exit?

This is an issue. I will fix it in the next version.

>
> > +        colo_record_bitmap(p->block, p->normal, p->normal_num);
> > +        colo_record_bitmap(p->block, p->zero, p->zero_num);
> > +    }
> > +}
> > +
> > +void multifd_colo_process_recv(MultiFDRecvParams *p)
> > +{
> > +    if (!migration_incoming_in_colo_state()) {
> > +        for (int i = 0; i < p->normal_num; i++) {
> > +            void *guest = p->block->host + p->normal[i];
> > +            void *cache = p->host + p->normal[i];
> > +            memcpy(guest, cache, multifd_ram_page_size());
> > +        }  
> 
> I see some differences between what ram.c does and what multifd will do
> after this patch regarding which flags are checked and order of copies
> (code below):
> 
> ram.c:
> 
>   - migration_incoming_colo_enabled && migration_incoming_in_colo_state:
>   Reads from stream into colo_cache.
>   
>   - migration_incoming_colo_enabled && !migration_incoming_in_colo_state:
>   Reads from stream into guest and then memcpy into colo_cache.
> 
>   - !migration_incoming_colo_enabled
>   Reads from stream into guest.
> 
> multifd.c:
> 
>   - migrate_colo:
>   Reads from stream into colo_cache.
>   
>   - !migration_incoming_in_colo_state:
>   memcpy from colo_cache into guest.
> 
>   - !migration_incoming_colo_enabled
>   ???
> 
> The resulting state should be the same, but I wonder if we want to i) use
> the same checks in multifd

migration_incoming_colo_enabled() shouldn't even exist anymore, so I'm
not using it here. migrate_colo() is much easier to reason about.

> and ii) when not in colo state, copy first
> into guest (using readv) and later memcpy into the colo_cache.

I think it is easier the way it is now.

> 
> ---
> ram.c:
> 
> host = host_from_ram_block_offset(block, addr);
> if (migration_incoming_colo_enabled()) {
>     if (migration_incoming_in_colo_state()) {
>         host = colo_cache_from_block_offset(block, addr, true);
>     } else {
>         host_bak = colo_cache_from_block_offset(block, addr, false);
>     }
> }
> qemu_get_buffer(f, host, TARGET_PAGE_SIZE);
> if (host_bak) {
>     memcpy(host_bak, host, TARGET_PAGE_SIZE);
> }
> 
> multifd:
> 
> if (migrate_colo()) {
>     p->host = p->block->colo_cache;
> }
> 
> for (int i = 0; i < p->normal_num; i++) {
>     p->iov[i].iov_base = p->host + p->normal[i];
> }
> return qio_channel_readv_all(p->c, p->iov, p->normal_num, errp);
> 
> if (!migration_incoming_in_colo_state()) {
>     for (int i = 0; i < p->normal_num; i++) {
>         void *guest = p->block->host + p->normal[i];
>         void *cache = p->host + p->normal[i];
>         memcpy(guest, cache, multifd_ram_page_size());
>     }
> }
> ---
> 
> > +        for (int i = 0; i < p->zero_num; i++) {
> > +            void *guest = p->block->host + p->zero[i];
> > +            memset(guest, 0, multifd_ram_page_size());
> > +        }  
> 
> At multifd_nocomp_recv, there will be a call to
> multifd_recv_zero_page_process(), which by that point will have p->host
> == p->block->colo_cache, so it looks like that function will do some
> zero page processing in the colo_cache, setting the rb->receivedmap for
> pages in the colo_cache and potentially also doing a memcpy. Is this
> intended?

rb->receivedmap is only for postcopy, right? So it doesn't apply with
colo.

> 
> I'm thinking that maybe it would overall be better to hook colo directly
> in to multifd_nocomp_recv:

But then it will only work for nocomp, right? It feels like the wrong
level of abstraction to me.

> 
> static int multifd_nocomp_recv(MultiFDRecvParams *p, Error **errp)
> {
>     uint32_t flags;
> 
>     if (migrate_mapped_ram()) {
>         return multifd_file_recv_data(p, errp);
>     }
> 
>     flags = p->flags & MULTIFD_FLAG_COMPRESSION_MASK;
> 
>     if (flags != MULTIFD_FLAG_NOCOMP) {
>         error_setg(errp, "multifd %u: flags received %x flags expected %x",
>                    p->id, flags, MULTIFD_FLAG_NOCOMP);
>         return -1;
>     }
> 
> +    if (migration_incoming_colo_enabled() && migration_incoming_in_colo_state()) {
> +        p->host = p->block->colo_cache;
> +    } // or else{}, depending on how deal with zero pages in the cache
> 
>     multifd_recv_zero_page_process(p);
> 
>     if (!p->normal_num) {
>         return 0;
>     }
> 
>     for (int i = 0; i < p->normal_num; i++) {
>         p->iov[i].iov_base = p->host + p->normal[i];
>         p->iov[i].iov_len = multifd_ram_page_size();
>         ramblock_recv_bitmap_set_offset(p->block, p->normal[i]);
>     }
> +    ret = qio_channel_readv_all(p->c, p->iov, p->normal_num, errp);
> +    if (ret != 0) {
> +        return ret;
> +    }
> +
> +    if (migration_incoming_colo_enabled()) {
> +        multifd_colo_process_recv();
> +    }
> 
>     return ret;
> }
> 
> 
> > +    }
> > +}
> > diff --git a/migration/multifd-colo.h b/migration/multifd-colo.h
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..82eaf3f48c47de2f090f9de52f9d57a337d4754a
> > --- /dev/null
> > +++ b/migration/multifd-colo.h
> > @@ -0,0 +1,26 @@
> > +/*
> > + * SPDX-License-Identifier: GPL-2.0-or-later
> > + *
> > + * multifd colo header
> > + *
> > + * Copyright (c) Lukas Straub <lukasstraub2@web.de>
> > + *
> > + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> > + * See the COPYING file in the top-level directory.
> > + */
> > +
> > +#ifndef QEMU_MIGRATION_MULTIFD_COLO_H
> > +#define QEMU_MIGRATION_MULTIFD_COLO_H
> > +
> > +#ifdef CONFIG_REPLICATION
> > +
> > +void multifd_colo_prepare_recv(MultiFDRecvParams *p);
> > +void multifd_colo_process_recv(MultiFDRecvParams *p);
> > +
> > +#else
> > +
> > +static inline void multifd_colo_prepare_recv(MultiFDRecvParams *p) {}
> > +static inline void multifd_colo_process_recv(MultiFDRecvParams *p) {}
> > +
> > +#endif
> > +#endif
> > diff --git a/migration/multifd-nocomp.c b/migration/multifd-nocomp.c
> > index 9be79b3b8e00371ebff9e112766c225bec260bf7..9f7a792fa761b3bc30b971b35f464103a61787f0 100644
> > --- a/migration/multifd-nocomp.c
> > +++ b/migration/multifd-nocomp.c
> > @@ -16,6 +16,7 @@
> >  #include "file.h"
> >  #include "migration-stats.h"
> >  #include "multifd.h"
> > +#include "multifd-colo.h"
> >  #include "options.h"
> >  #include "migration.h"
> >  #include "qapi/error.h"
> > @@ -269,7 +270,6 @@ int multifd_ram_unfill_packet(MultiFDRecvParams *p, Error **errp)
> >          return -1;
> >      }
> >  
> > -    p->host = p->block->host;
> >      for (i = 0; i < p->normal_num; i++) {
> >          uint64_t offset = be64_to_cpu(packet->offset[i]);
> >  
> > @@ -294,6 +294,14 @@ int multifd_ram_unfill_packet(MultiFDRecvParams *p, Error **errp)
> >          p->zero[i] = offset;
> >      }
> >  
> > +    if (migrate_colo()) {
> > +        multifd_colo_prepare_recv(p);
> > +        assert(p->block->colo_cache);
> > +        p->host = p->block->colo_cache;  
> 
> Can't you just use p->block->colo_cache later? I don't see why p->host
> needs to be set beforehand even in the non-colo case.

We should not touch the guest ram directly while in colo state, since
the incoming guest is running and we either want to receive and apply a
whole checkpoint with all ram into colo cache and all device state,
or if anything goes wrong during checkpointing, keep the currently
running guest on the incoming side in pristine state.

I have written more about colo migration here:

https://lore.kernel.org/qemu-devel/20260117204913.584e1829@penguin/
https://lore.kernel.org/qemu-devel/aXE1i9xJ81EWokYz@x1.local/

> 
> > +    } else {
> > +        p->host = p->block->host;
> > +    }
> > +
> >      return 0;
> >  }
> >  
> > diff --git a/migration/multifd.c b/migration/multifd.c
> > index 332e6fc58053462419f3171f6c320ac37648ef7b..220ed8564960fdabc58e4baa069dd252c8ad293c 100644
> > --- a/migration/multifd.c
> > +++ b/migration/multifd.c
> > @@ -29,6 +29,7 @@
> >  #include "qemu-file.h"
> >  #include "trace.h"
> >  #include "multifd.h"
> > +#include "multifd-colo.h"
> >  #include "options.h"
> >  #include "qemu/yank.h"
> >  #include "io/channel-file.h"
> > @@ -1258,6 +1259,13 @@ static int multifd_ram_state_recv(MultiFDRecvParams *p, Error **errp)
> >      int ret;
> >  
> >      ret = multifd_recv_state->ops->recv(p, errp);
> > +    if (ret != 0) {
> > +        return ret;
> > +    }
> > +
> > +    if (migrate_colo()) {
> > +        multifd_colo_process_recv(p);
> > +    }  
> 
> Either put all of colo hooks in multifd.c or multifd-nocomp.c. I think
> the latter is more appropriate as we have mapped_ram already in
> there. Let's drop patch 3 and put this in multifd_nocomp_recv().

Again, it also should work with compression and multifd_nocomp_recv()
is for nocomp only.

> 
> >  
> >      return ret;
> >  }
> > diff --git a/migration/multifd.h b/migration/multifd.h
> > index 89a395aef2b09a6762c45b5361e0ab63256feff6..fbc35702b062fdc3213ce92baed35994f5967c2b 100644
> > --- a/migration/multifd.h
> > +++ b/migration/multifd.h
> > @@ -279,7 +279,10 @@ typedef struct {
> >      uint64_t packets_recved;
> >      /* ramblock */
> >      RAMBlock *block;
> > -    /* ramblock host address */
> > +    /*
> > +     * Normally, it points to ramblock's host address.  When COLO
> > +     * is enabled, it points to the mirror cache for the ramblock.
> > +     */
> >      uint8_t *host;
> >      /* buffers to recv */
> >      struct iovec *iov;  


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