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[Xen-devel] RFC v1: Xen block protocol overhaul - problem statement (with pictures!)


I am including some folks that are not always on Xen-devel to see if they have
some extra ideas or can correct my misunderstandings.

This is very much RFC - so there is bound to be some bugs.
The original is here
in case one wishes to modify/provide comment on that one.

There are outstanding issues we have now with the block protocol:
Note: I am assuming 64-bit guest/host - as the sizeâs of the structures
change on 32-bit. I am also attaching the header for the blkif ring
as of today.

A) Segment size is limited to 11 pages. It means we can at most squeeze
in 44kB per request. The ring can hold 32 (next power of two below 36)
requests, meaning we can do 1.4M of outstanding requests.

B). Producer and consumer index is on the same cache line. In present hardware
that means the reader and writer will compete for the same cacheline causing a
ping-pong between sockets.

C). The requests and responses are on the same ring. This again causes the
ping-pong between sockets as the ownership of the cache line will shift between

D). Cache alignment. Currently the protocol is 16-bit aligned. This is awkward
as the request and responses sometimes fit within a cacheline or sometimes
straddle them.

E). Interrupt mitigation. We are currently doing a kick whenever we are
done âprocessingâ the ring. There are better ways to do this - and
we could use existing network interrupt mitigation techniques to make the
code poll when there is a lot of data. 

F). Latency. The processing of the request limits us to only do 44kB - which 
that a 1MB chunk of data - which on contemporary devices would only use I/O 
- would be split up in multiple ârequestsâ inadvertently delaying the 
processing of
said block. 

G) Future extensions. DIF/DIX for integrity. There might
be other in the future and it would be good to leave space for extra
flags TBD. 

H). Separate the response and request rings. The current
implementation has one thread for one block ring. There is no reason why
there could not be two threads - one for responses and one for requests -
and especially if they are scheduled on different CPUs. Furthermore this
could also be split in multi-queues - so two queues (response and request)
on each vCPU. 

I). We waste a lot of space on the ring - as we use the
ring for both requests and responses. The response structure needs to
occupy the same amount of space as the request structure (112 bytes). If
the request structure is expanded to be able to fit more segments (say
the âstruct blkif_sring_entry is expanded to ~1500 bytes) that still
requires us to have a matching size response structure. We do not need
to use that much space for one response. Having a separate response ring
would simplify the structures. 

J). 32 bit vs 64 bit. Right now the size
of the request structure is 112 bytes under 64-bit guest and 102 bytes
under 32-bit guest. It is confusing and furthermore requires the host
to do extra accounting and processing.

The crude drawing displays memory that the ring occupies in offset of
64 bytes (cache line). Of course future CPUs could have different cache
lines (say 32 bytes?)- which would skew this drawing. A 32-bit ring is
a bit different as the âstruct blkif_sring_entryâ is of 102 bytes.

The A) has two solutions to this (look at
http://comments.gmane.org/gmane.comp.emulators.xen.devel/140406 for
details). One proposed by Justin from Spectralogic has to negotiate
the segment size. This means that the âstruct blkif_sring_entryâ
is now a variable size. It can expand from 112 bytes (cover 11 pages of
data - 44kB) to 1580 bytes (256 pages of data - so 1MB). It is a simple
extension by just making the array in the request expand from 11 to a
variable size negotiated.

The math is as follow.

        struct blkif_request_segment {
                uint32 grant;                         // 4 bytes uint8_t
                first_sect, last_sect;// 1, 1 = 6 bytes
(6 bytes for each segment) - the above structure is in an array of size
11 in the request. The âstruct blkif_sring_entryâ is 112 bytes. The
change is to expand the array - in this example we would tack on 245 extra
âstruct blkif_request_segmentâ - 245*6 + 112 = 1582. If we were to
use 36 requests (so 1582*36 + 64) we would use 57016 bytes (14 pages).

The other solution (from Intel - Ronghui) was to create one extra
ring that only has the âstruct blkif_request_segmentâ in them. The
âstruct blkif_requestâ would be changed to have an index in said
âsegment ringâ. There is only one segment ring. This means that the
size of the initial ring is still the same. The requests would point
to the segment and enumerate out how many of the indexes it wants to
use. The limit is of course the size of the segment. If one assumes a
one-page segment this means we can in one request cover ~4MB. The math
is as follow:

First request uses the half of the segment ring - so index 0 up
to 341 (out of 682). Each entry in the segment ring is a âstruct
blkif_request_segmentâ so it occupies 6 bytes. The other requests on
the ring (so there are 35 left) can use either the remaining 341 indexes
of the sgement ring or use the old style request. The old style request
can address use up to 44kB. For example:

 sring[0]->[uses 0->341 indexes in the segment ring] = 342*4096 = 1400832
 sring[1]->[use sthe old style request] = 11*4096 = 45056
 sring[2]->[uses 342->682 indexes in the segment ring] = 1392640
 sring[3..32] -> [uses the old style request] = 29*4096*11 = 1306624

Total: 4145152 bytes Naturally this could be extended to have a bigger
segment ring to cover more.

The problem with this extension is that we use 6 bytes and end up
straddling a cache line. Using 8 bytes will fix the cache straddling. This
would mean fitting only 512 segments per page.

There is yet another mechanism that could be employed  - and it borrows
from VirtIO protocol. And that is the âindirect descriptorsâ. This
very similar to what Intel suggests, but with a twist.

We could provide a new BLKIF_OP (say BLKIF_OP_INDIRECT), and the âstruct
blkif_sringâ (each entry can be up to 112 bytes if needed - so the
old style request would fit). It would look like:

/* so 64 bytes under 64-bit. If necessary, the array (seg) can be
expanded to fit 11 segments as the old style request did */ struct
blkif_request_indirect {
        uint8_t        op;           /* BLKIF_OP_* (usually READ or WRITE    */
// 1 blkif_vdev_t   handle;       /* only for read/write requests         */ // 
#ifdef CONFIG_X86_64
        uint32_t       _pad1;             /* offsetof(blkif_request,u.rw.id) == 
8 */ // 2
        uint64_t       id;           /* private guest value, echoed in resp  */
        grant_ref_t    gref;        /* reference to indirect buffer frame  if 
            struct blkif_request_segment_aligned seg[4]; // each is 8 bytes
} __attribute__((__packed__));

struct blkif_request {
        uint8_t        operation;    /* BLKIF_OP_???  */
        union {
                struct blkif_request_rw rw;
                struct blkif_request_indirect
                indirect; â other ..
        } u;
} __attribute__((__packed__));

The âoperationâ would be BLKIF_OP_INDIRECT. The read/write/discard,
etc operation would now be in indirect.op. The indirect.gref points to
a page that is filled with:

struct blkif_request_indirect_entry {
        blkif_sector_t sector_number;
        struct blkif_request_segment seg;
} __attribute__((__packed__));
//16 bytes, so we can fit in a page 256 of these structures.

This means that with the existing 36 slots in the ring (single page)
we can cover: 32 slots * each blkif_request_indirect covers: 256 * 4096
~= 32M. If we donât want to use indirect descriptor we can still use
up to 4 pages of the request (as it has enough space to contain four
segments and the structure will still be cache-aligned).

B). Both the producer (req_* and rsp_*) values are in the same
cache-line. This means that we end up with the same cacheline being
modified by two different guests. Depending on the architecture and
placement of the guest this could be bad - as each logical CPU would
try to write and read from the same cache-line. A mechanism where
the req_* and rsp_ values are separated and on a different cache line
could be used. The value of the correct cache-line and alignment could
be negotiated via XenBus - in case future technologies start using 128
bytes for cache or such. Or the the producer and consumer indexes are in
separate rings. Meaning we have a ârequest ringâ and a âresponse
ringâ - and only the âreq_prodâ, âreq_eventâ are modified in
the ârequest ringâ. The opposite (resp_*) are only modified in the
âresponse ringâ.

C). Similar to B) problem but with a bigger payload. Each
âblkif_sring_entryâ occupies 112 bytes which does not lend itself
to a nice cache line size. If the indirect descriptors are to be used
for everything we could âslim-downâ the blkif_request/response to
be up-to 64 bytes. This means modifying BLKIF_MAX_SEGMENTS_PER_REQUEST
to 5 as that would slim the largest of the structures to 64-bytes.
Naturally this means negotiating a new size of the structure via XenBus.

D). The first picture shows the problem. We now aligning everything
on the wrong cachelines. Worst in â of the cases we straddle
three cache-lines. We could negotiate a proper alignment for each
request/response structure.

E). The network stack has showed that going in a polling mode does improve
performance. The current mechanism of kicking the guest and or block
backend is not always clear.  [TODO: Konrad to explain it in details]

F). The current block protocol for big I/Os that the backend devices can
handle ends up doing extra work by splitting the I/O in smaller chunks,
and then reassembling them. With the solutions outlined in A) this can
be fixed. This is easily seen with 1MB I/Os. Since each request can
only handle 44kB that means we have to split a 1MB I/O in 24 requests
(23 * 4096 * 11 = 1081344). Then the backend ends up sending them in
sector-sizes- which with contemporary devices (such as SSD) ends up with
more processing. The SSDs are comfortable handling 128kB or higher I/Os
in one go.

G). DIF/DIX. This a protocol to carry extra âchecksumâ information
for each I/O. The I/O can be a sector size, page-size or an I/O size
(most popular are 1MB). The DIF/DIX needs 8 bytes of information for
each I/O. It would be worth considering putting/reserving that amount of
space in each request/response. Also putting in extra flags for future
extensions would be worth it - however the author is not aware of any
right now.

H). Separate response/request. Potentially even multi-queue per-VCPU
queues. As v2.6.37 demonstrated, the idea of WRITE_BARRIER was
flawed. There is no similar concept in the storage world were the
operating system can put a food down and say: âeverything before this
has to be on the disk.â There are ligther versions of this - called
âFUAâ and âFLUSHâ. Depending on the internal implementation
of the storage they are either ignored or do the right thing. The
filesystems determine the viability of these flags and change writing
tactics depending on this. From a protocol level, this means that the
WRITE/READ/SYNC requests can be intermixed - the storage by itself
determines the order of the operation. The filesystem is the one that
determines whether the WRITE should be with a FLUSH to preserve some form
of atomicity. This means we do not have to preserve an order of operations
- so we can have multiple queues for request and responses. This has
show in the network world to improve performance considerably.

I). Wastage of response/request on the same ring. Currently each response
MUST occupy the same amount of space that the request occupies - as the
ring can have both responses and requests. Separating the request and
response ring would remove the wastage.

J). 32-bit vs 64-bit (or 102 bytes vs 112 bytes). The size of the ring
entries is different if the guest is in 32-bit or 64-bit mode. Cleaning
this up to be the same size would save considerable accounting that the
host has to do (extra memcpy for each response/request).

Attachment: blkif_sring_struct.png
Description: PNG image

Attachment: blkif_segment_struct.png
Description: PNG image

Attachment: blkif.h
Description: Text document

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