Bulk index using .net HttpClient get TaskCanceledException


I have a middle tier that keep fetching data from data source and do some processing work then put them into elasticsearch. I found that when the middle tier machine (it is a Azure Service Fabric VM) is hot (CPU goes up to 95%), the bulk index response from elasticsearch sometimes take longer time, up to 30 seconds (normally 1~4 seconds) or more, and sometimes it get TaskCanceledException. I'm using .net framework and HttpClient to send request, and I set the time out value to be 45 seconds.

Does anyone know what may be the cause?
Is it a networking issue caused by high CPU usage in the client machine?
Or is it because the elasticsearch cluster is too busy? Some elasticsearch data node CPU usage goes up to >90% when there is exception, but the exception does not always happen when the data node has high CPU usage.

Thanks for your help in advanced. Here is my code sample:

public void MyTask()
using (HttpClient httpClient = new HttpClient(new WebRequestHandler()))
httpClient.DefaultRequestHeaders.TransferEncodingChunked = false;
httpClient.DefaultRequestHeaders.Add("Connection", "keep-alive");
httpClient.Timeout = TimeSpan.FromSeconds(30);

    while (true)
        // fetch items from the mem queue. these items are already pre-processed
        var logLines = FetchLogLines();
        List<Task<long>> sendTasks = new List<Task<long>>();
        sendTasks.Add(Task.Run(() => SendBulkInsertRequest(httpClient, logLines, IndexName)));


public long SendBulkInsertRequest(HttpClient httpClient, List loglines, string indexName)
// generate the post body
StringBuilder sb = new StringBuilder();
HttpResponseMessage result = null;

string header = "{\"index\":{}}";

foreach (var log in loglines)

var postBody = sb.ToString();
var content = new StringContent(postBody, Encoding.UTF8, "application/json");

// generate request url
string indexUrl = GetIndexUrl();
int sentCount = 0;

// send index request
    result = httpClient.PostAsync(indexUrl, content).Result;
    sentCount = result.IsSuccessStatusCode ? loglines.Count : 0;
catch (Exception ex)
    this.Logger($"IndexTask-{taskId}: exception while bulk insert to url: {indexUrl}", ex);               
    sentCount = 0;

return sentCount;


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