Date: Sat, 25 Mar 2023 12:12:12 +0800
From: Tao Liu <ltao@redhat.com>
To: crash-utility@redhat.com
Subject: [Crash-utility] [PATCH 0/5] [RFC] Multi-thread support for
        search cmd
Message-ID: <20230325041217.8184-1-ltao@redhat.com>
Content-Type: text/plain; charset="US-ASCII"; x-default=true

The primary part of the patchset will introduce multithread support for search
cmd to improve its performance. A search operation is mainly made up with 2

steps: 1) readmem data into pagebuf, 2) search specific values within the
pagebuf. A typical workflow of search is as follows:

for addr from low to high:
do
        readmem(addr, pagebuf)
        search_value(value, pagebuf)
        addr += pagesize
done

There are 2 points which we can accelerate: 1) readmem don't have to wait
search_value, when search_value is working, readmem can read the next pagebuf
at the same time. 2) higher addr don't have to wait lower addr, they can be
processed at the same time if we carefully arrange the output order.

For point 1, we introduce zones for pagebuf, e.g. search_value can work on
zone 0 while readmem can prepare the data for zone 1. For point 2, we introduce
multiple search_value in threads, e.g. readmem will prepare 100 pages as a
batch, then we will have 4 threads of search_value, thread 0 handles page 1~25,
thread 2 handles page 26~50 page, thread 3 handles page 51~75, thread 4 handles
page 76~100.

A typical workflow of multithread search implemented in this patchset is as
follows, which removed thread synchronization:

pagebuf[ZONE][BATCH]
zone_index = buf_index = 0
create_thread(4, search_value)
for addr from low to high:
do
        if buf_index < BATCH
                readmem(addr, pagebuf[zone_index][buf_index++])
                addr += pagesize
        else
                start_thread(pagebuf[zone_index], 0/4 * BATCH, 1/4 * BATCH)
                start_thread(pagebuf[zone_index], 1/4 * BATCH, 2/4 * BATCH)
                start_thread(pagebuf[zone_index], 2/4 * BATCH, 3/4 * BATCH)
                start_thread(pagebuf[zone_index], 3/4 * BATCH, 4/4 * BATCH)
                zone_index++
                buf_index = 0
        fi
done

readmem works in the main process and not multi-threaded, because readmem will
not only read data from vmcore, decompress it, but walk through page tables if
virtual address given. It is hard to reimplement it into thread safe version,
search_value is easier to be made thread-safe. By carefully choose batch size
and thread num, we can maximize the concurrency.

The last part of the patchset, is replacing lseek/read to pread for kcore and
diskdumped vmcore.

Here is the performance test result chart. Please note the vmcore and
kcore are tested seperately on 2 different machines. crash-orig is the
crash compiled with clean upstream code, crash-pread is the code with only
pread patch applied(patch 5), crash-multi is the code with only multithread
patches applied(patch 1~4).

ulong search:

    $ time echo "search abcd" | ./crash-orig vmcore vmlinux > /dev/null
    $ time echo "search abcd -f 4 -n 4" | ./crash-multi vmcore vmlinux > /dev/null

                         45G vmcore                             64G kcore
                real        user        sys             real       user       sys
crash-orig      16m56.595s  15m57.188s  0m56.698s       1m37.982s  0m51.625s  0m46.266s
crash-pread     16m46.366s  15m55.790s  0m48.894s       1m9.179s   0m36.646s  0m32.368s
crash-multi     16m26.713s  19m8.722s   1m29.263s       1m27.661s  0m57.789s  0m54.604s

string search:

    $ time echo "search -c abcddbca" | ./crash-orig vmcore vmlinux > /dev/null
    $ time echo "search -c abcddbca -f 4 -n 4" | ./crash-multi vmcore vmlinux > /dev/null

                        45G vmcore                              64G kcore
                real        user        sys             real       user       sys
crash-orig      33m33.481s  32m38.321s  0m52.771s       8m32.034s  7m50.050s  0m41.478s
crash-pread     33m25.623s  32m35.019s  0m47.394s       8m4.347s   7m35.352s  0m28.479s
crash-multi     16m31.016s  38m27.456s  1m11.048s       5m11.725s  7m54.224s  0m44.186s

Discussion:

1) Either multithread and pread patches can improve the performance a
   bit, so if both patches applied, the performance can be better.

2) Multi-thread search performs much better in search time consumptive
   tasks, such as string search.

Tao Liu (5):
  Introduce multi-thread to search_virtual
  Introduce multi-thread to search_physical
  Introduce multi-thread to string search
  Introduce multi-thread options to search cmd
  Replace lseek/read into pread for kcore and vmcore reading.

 defs.h     |    6 +
 diskdump.c |   11 +-
 help.c     |   17 +-
 memory.c   | 1176 +++++++++++++++++++++++++++++++++++++++++-----------
 netdump.c  |    5 +-
 task.c     |   14 +
 6 files changed, 969 insertions(+), 260 deletions(-)

--
2.33.1